Write down what the next person needs

`docs/lane-handoff.md`, after `timing-handoff.md`'s shape, because the next
thread starts cold and the expensive part of that is not the code — it is the
decisions that were argued out and would otherwise be argued again.

So the section that matters most is the one listing what NOT to re-litigate:
the shot length is authored, placing ripples and overwrite is blank-then-place,
a position inside a cel refuses and names split, a correction has no time space
of its own, a layer's values are a channel, a conflict is not a problem, and a
command refuses rather than guesses. Each of those is a paragraph here and a
commit message in full.

Then the vocabulary, since it was settled one commit ago and the old words are
still in this repository's history: instance, cel, lane, drawing, and placement
for where a node sits only. With the warning that `exposure` still means the
`:expose` grid and always did.

Then the mechanisms that keep paying out — `:span` in the node's own frames
above all, which is why split, trim and blank cost almost nothing — the known
gaps, and how to run the suites, including that a release build clobbers the dev
bundle the browser tests need.

Recommended next piece is the cel sheet: it needs no new model, and it is the
first real evidence the document is not shaped by the timeline that grew up
with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/lane-handoff.md

@@ -0,0 +1,190 @@
+# Lane and cel handoff
+
+Status (2026-09-30): the lane model is implemented through its commands. Cels are
+ordinary nodes with their own playback clock, a lane draws them as one row, the
+cel commands all exist, and correction layers evaluate and survive regeneration.
+What is missing is a second view and the commands that make a correction.
+
+Seven commits on branch `lanes`, off `624242b`. `master` is untouched and can be
+fast-forwarded. Each commit message is the argument for its change and is worth
+reading before touching what it did — they are the design record, more than this
+file is.
+
+    3d3c1bb  An occurrence is a node, with a clock of its own
+    9446829  Reuse, duplicate and make unique: deciding what is shared
+    26517af  A position is an argument, not another command
+    94c0a21  A correction is a layer, and a layer's values are a channel
+    72b57e3  Regenerate the base, keep the hand work, and say when you cannot
+    76106d3  The shot is as long as somebody said it was
+    598c186  One word for one thing: it is a cel
+
+## Read first, in this order
+
+1. [The Lane Model](lane-model.md) — the design, and the status note under
+   *Proof obligations* says what is built. It supersedes `animation-model.md`,
+   `timing-model.md` and `architecture.md` wherever they overlap.
+2. `frontend/src/arthur/domain/lane.cljs` — every command, and the reasoning in
+   its docstrings.
+3. `frontend/test/arthur/domain/lane_test.cljs` — what the model is asserted to
+   do. It is the fastest way to see the shapes.
+4. `frontend/src/arthur/domain/channel.cljs`, the correction-layer section.
+
+## Vocabulary — one word for one thing
+
+Renamed in `598c186`, after four words had accumulated for one object. Use these
+and do not reintroduce the others.
+
+| word | means |
+| --- | --- |
+| instance | the `:kind`. The general thing, anywhere in a document |
+| cel | an instance in a lane. One drawing, held for some duration |
+| lane | a group with `:layout :sequence` |
+| drawing | the content a cel names — an ordinary symbol |
+| placement | ONLY where a node sits: `nest/placement`, and the transform that puts a face on the stage. Never the node itself |
+
+`occurrence` and `exposure` are not words for a cel. **`exposure` means something
+else and still does**: `:time :expose` is how many frames each step of a subtree
+lasts, which is what shooting on twos is — `node/expose`, `clock/exposed-frame`,
+`subs/render ::exposure`. Keeping these apart is why the block is called a cel.
+
+`:layout :sequence` stays as the field, and is the one place two words are kept
+on purpose: the layout names the RULE — children follow one another and may not
+overlap — and a group carrying it is called a lane. `node/lane?` is where they
+meet.
+
+The second view is the CEL SHEET, not the exposure sheet.
+
+## Decisions already made — do not re-litigate
+
+These were each argued out and are load-bearing. Changing one is a design
+decision, not a cleanup.
+
+- **The shot length is authored.** `:frames` is the symbol's window; the
+  occupied extent of its lanes is a different fact derived from the cels. A
+  command grows the window only when the caller passes `:extent :grow-symbol`,
+  and never shrinks it. Blanking the end of a shot leaves empty frames at the
+  end, because deriving the window from the extent would make deleting the last
+  drawing silently shorten the film. `lane/finish`.
+- **Placing ripples; overwrite is `blank` then `place`.** Overwrite is not a
+  policy argument and should not become one until there is a reason `blank`
+  cannot serve. An argument whose second value is unimplemented is worse than
+  no argument.
+- **A position inside a cel refuses and names `split`.** One command must not
+  quietly perform two. The UI offers the retry.
+- **A correction has no time space of its own.** Its `:support` and its values'
+  keys are in the frames the base channel's keys are in — the node's. A
+  correction on a lane is in lane frames and reaches across the drawings under
+  it; one on a cel travels with that cel. Ownership already answered it.
+- **A layer's values are a channel.** Constant, ramp and return motion are one
+  mechanism. Do not add a second way to say what a value is over time.
+- **A conflict is not a `problem`.** A document whose topology outgrew a
+  correction loads, evaluates and saves; `clip/conflicts` lists the decisions
+  waiting for a person. `problems` means the document will not load.
+- **Refuse rather than guess.** Every command returns `{:clip :selection}` or
+  `{:refused why}`, never a half-applied edit. Where the model needs a choice
+  nobody has made, refusing and saying why is the behaviour, not a placeholder.
+- **A cel is not a row.** Rows, expansion and selection are editor state. The
+  document has never known about rows and must not learn.
+
+## Next steps, in order
+
+1. **The cel sheet.** The doc's own test that the document is separate from its
+   presentation: frames down, a column per lane, one cell per frame naming the
+   drawing. It needs NO new model — `symbol/lane-cels` and the existing commands
+   are the whole API. The obligation is that the same command issued from the
+   sheet and from the timeline produces identical document changes and the same
+   selection. This is the recommended next piece: it is self-contained, it is
+   specified, and it is the first real evidence the model is not shaped by the
+   timeline that grew up with it.
+2. **The commands that make a correction** — Constant adjustment, Ramp, Return
+   motion over a selected range, per `lane-model.md`. The evaluator is done and
+   has no opinion about how a range or a motion shape is chosen, which is now a
+   view question. A panel also needs to offer `clip/conflicts` for resolution.
+   Note the one open question: a correction needs a stable `:id` from somewhere,
+   and cel ids come from the caller because this namespace is pure.
+3. **Slip source and retime.** Both have real design questions open and the doc
+   says to refuse rather than approximate: retime needs a defined warp and
+   interpolation behaviour, and is not moving keys whose numbers happen to fall
+   inside a selection.
+4. **Deleting reused content.** Reference discovery exists (`node/sources`,
+   `clip/places`, `clip/contains-symbol?`); the policy does not.
+5. **Collaboration.** `lane-model.md` is explicit that one leaf per channel does
+   NOT solve two people editing different keys of the same channel. No conflict
+   policy exists for that.
+
+## Mechanisms to reuse — these keep paying out
+
+- **`:span` is in the node's OWN frames** and `:time` says where they land in
+  the lane. Moving an edge of a cel is therefore one write to `:span`, with
+  `:time` and `:playback` untouched. This is why split costs nothing, why the
+  two halves of a split go on meaning what the one cel meant, why trimming the
+  front of a playing insert starts it later into its animation instead of
+  restarting it, and why extending a hold leaves lane keys alone. `lane/local`
+  and `lane/edged` are the whole geometry; trim, split and blank are all it.
+- **`lane/finish`** is the one commit path: it validates, applies the shot-length
+  policy, and returns the refusal. New commands go through it.
+- **`:required-frames` plus the retry event** is the pattern for "this needs a
+  decision you have not made": the domain reports what it would need, the UI
+  offers one button. `events/ui/lane-retry`.
+- **`lane/lane-frame`** converts a symbol frame to a lane frame, or returns nil
+  through a stepped or looping lane where there is no single answer. Nil refuses;
+  it never snaps.
+- **`channel/conflict-with`** is the single rule for "can this layer apply to
+  this base", used by the validator, by `conflicts`, and by the regeneration
+  that marks them.
+- **Two test patterns worth copying.** `the-cursor-agrees-with-the-specification-in-any-frame-order`
+  holds the optimized cursor to `value-at` in forward, backward and random order
+  — add a case to it for any new channel shape. And `drawn` in `lane_test`
+  samples every frame before and after an edit, which is how split and trim are
+  proved to change nothing: state a claim as "the same picture" rather than as
+  numbers computed by hand.
+
+## Known gaps and traps
+
+- **Audio lanes do not work.** `symbol/lane-problems` requires `:instance`
+  children, so an audio node in a lane is rejected outright. `lane-model.md`
+  says a lane may hold visual OR audio cels and should reject only a mixture.
+- **`:z` is required on cels and means nothing there.** A lane never has two
+  cels on one frame, so draw order between them cannot matter. `node/problems`
+  requires `:z` on every node uniformly, which is its own kind of simplicity —
+  but the field is noise on a cel.
+- **`channel/offset-onto` throws** on a shape mismatch that no regeneration has
+  recorded as a conflict. That is deliberate — a correction that silently does
+  not take is the failure the design exists to prevent, and `channel/problems`
+  catches the authored case — but it is a throw in the read path, so any new
+  producer of layers must not create a mismatched one.
+- **`docs/timing-handoff.md` is a separate, unreconciled thread.** Performance-
+  pose selection and plate drawings/tracing, instance-specific picture-rate
+  requests, `pose/put-cut` addressing only `:main`. It predates the lane model
+  and nobody has squared the two.
+- **The button row in the timeline pane is a test harness, not a design.** It is
+  how the commands were made reachable and provable. `lane-model.md` describes
+  the real cel action strip, the breadcrumb and the location bar; none exist.
+- **`shadow-cljs release app` clobbers the dev bundle.** Both builds write
+  `../static/arthur/js`, which Django serves, and the optimized build does not
+  export the `arthur` global — so after a release the browser tests fail with
+  `ReferenceError: arthur is not defined`. Run `npx shadow-cljs compile app` to
+  restore it. A running `watch app` does not notice; it rebuilds on the next
+  source change.
+
+## Running it
+
+From `frontend/`:
+
+    npx shadow-cljs compile test && node out/node-tests.js     # 424 tests, 5,749 assertions
+    npx shadow-cljs compile app                                # the bundle Django serves
+    npx shadow-cljs release app                                # then `compile app` again — see above
+
+The browser tests need the Django dev server up (`mise exec -- python manage.py
+runserver 8778` from the repo root) and a compiled dev bundle:
+
+    node --experimental-websocket test/browser/lane.mjs                  # the lane/cel flow
+    CHROME=/usr/bin/chromium node --experimental-websocket test/browser/take.mjs
+
+`take.mjs` defaults to a macOS Chrome path, hence `CHROME=`. It writes a real
+project to the local server by design; `lane.mjs` never writes to the server.
+
+From the repo root: `mise exec -- python manage.py test clips` — 56 tests.
+
+Documents are schema 3. A version 2 document is not read and nothing converts
+one; there is no backward compatibility to preserve anywhere in this work.

docs/lane-model.md

@@ -7,6 +7,9 @@ correction, overwrite, the retiming commands and the remaining views are not.
 See the status note under
 [Proof obligations](#proof-obligations-and-implementation-order).
 
+[Lane and cel handoff](lane-handoff.md) records what is built, the decisions
+that are settled, and what to do next.
+
 This revises the Claude artifact [The Lane Model](https://claude.ai/code/artifact/cd42981d-ed08-493f-94df-b7dd6657f0e6).
 Its prose and diagram source were recovered from session
 `1c603f71-84eb-498e-aeaf-4c0346f1f513`; the live artifact was not accessible for