A lane is a generic row of symbol clips

A lane was a drawing lane: the only thing that could go in one was a one-frame
held cel, and every other symbol instance stayed a permanent root row of its
own. Those are not two kinds of timing, they are one kind with two creation
policies. `lane/place-symbol` drops any library symbol in as a clip that plays
naturally at speed one, `lane/adopt` moves an instance that is already in the
document into a lane keeping its source, span, playback and corrections, and
`append-drawing`/`overwrite-drawing` keep being the policy that makes a new
empty symbol a one-frame hold. The child shape they produce is the same.

Both new commands claim their interval through `blank` before they write, so
the partition rule is unchanged and unduplicated: placing into occupied lane
time trims, removes or splits the incumbents, and a lane still never stores an
overlap. Real compositing overlap is another lane, where the order is explicit.

Creating a symbol with nothing aimed now makes a lane and a clip in it instead
of a loose root instance, and a pool drop prefers an explicitly targeted lane,
then the selected one, and makes a lane only when there is neither. That is
what stops the row-per-symbol growth coming back in through the drop path, and
it is why `add-lane` now takes a z in front of the existing root nodes and
calls what it makes a "lane" rather than "drawings".

The timeline learned the two gestures that a generic lane needs. A clip body
dragged over another lane's track previews there as a dashed block and lands
through `::adopt-in-lane`; the track is found with `elementsFromPoint` and its
selection read back off the element, because a pointer capture does not
retarget. A pool drop over an existing lane previews as a dashed clip inside
that lane instead of a temporary new row that appears and then vanishes --
which also needed the drag-leave check to be geometric, since inserting the
preview changes the element under the pointer and Chromium then reports a leave
with no related target. Lanes are renameable from their label, by double-click,
F2, or the pencil, through `::rename-node`.

`symbol/lane-cels` is `symbol/lane-clips`, and the vocabulary table in the
handoff now separates the two words it had merged: a clip is an instance in a
lane, and a cel is specifically the one-frame held source that drawing creation
makes. Keeping `cel` for the policy is what lets the lane stop being about
drawings at all.

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

docs/lane-handoff.md

@@ -1,12 +1,14 @@
-# Lane and cel handoff
+# Lane and symbol-clip handoff
 
-Status (2026-10-01): the timeline is the one timing interface. Cels are ordinary
-nodes with their own playback clock and appear as blocks on one row per drawing
-lane. Blocks move by mouse; edge drags trim without overlap; Shift-right-edge
-drags ripple every later cel; and the center of a shared cut composes the two
-edge edits into a rolling edit. Linked audio follows picture moves while its
-edges remain independently trimmable. Rotation and position corrections survive
-regeneration and expose conflicts for removal or retry.
+Status (2026-10-01): the timeline is the one timing interface. A lane is a
+generic non-overlapping row of symbol clips; it is not a special drawing type.
+Dropping a library symbol makes a naturally playing clip, while creating a new
+empty symbol makes a one-frame held clip at the playhead. With no destination
+lane, either operation creates one. Existing legacy root symbol rows can be
+dragged into a lane. Blocks move by mouse; edge drags claim time by trimming
+neighbors; Shift-edge drags ripple every later clip; and the center of a shared
+cut composes the two edge edits into a rolling edit. Linked audio follows picture
+moves while its edges remain independently trimmable.
 
 The commits beginning at `3d3c1bb` are the argument for the model and are worth
 reading before touching what they did — they are the design record, more than
@@ -39,9 +41,10 @@ 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 |
+| clip | an instance in a lane. It can hold one source frame or play a symbol naturally |
+| cel | specifically a one-frame source held over a clip's duration; the empty-symbol/drawing creation policy |
 | lane | a group with `:layout :sequence` |
-| drawing | the content a cel names — an ordinary symbol |
+| drawing | content authored into a symbol; not a different timeline node type |
 | 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
@@ -82,19 +85,29 @@ decision, not a cleanup.
 - **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
+- **A clip is not a row.** Rows, expansion and selection are editor state. The
   document has never known about rows and must not learn.
+- **A lane is generic.** Drawing creation, library placement, and adopting an
+  existing root instance all produce the same child instance shape. The only
+  difference is playback policy: a new empty drawing holds source frame zero;
+  a dropped library symbol plays at speed one.
+- **Placement claims time.** Lanes never store overlaps. A new or extended clip
+  trims, removes, or splits whatever previously owned the claimed interval.
+  Real compositing overlap uses another lane, where ordering remains explicit.
 
 ## Current timeline interaction
 
-- Creating a symbol inside an aimed drawing lane creates a one-frame cel at the
-  playhead. Drawing a polygon uses the existing cel there or creates the same
-  one-frame cel when the frame is empty.
-- Dragging a cel body moves it. A linked audio node follows a picture move;
+- Creating a symbol inside an aimed lane creates a one-frame held clip at the
+  playhead. With no aimed lane it first creates a lane. Drawing a polygon uses
+  the existing clip there or creates the same one-frame clip when the frame is
+  empty.
+- Dropping any library symbol into a lane creates a natural-duration playing
+  clip. Dropping it on unclaimed timeline or stage space first creates a lane.
+- Dragging a clip body moves it. A linked audio node follows a picture move;
   moving or trimming the audio itself remains independent.
 - Dragging a right edge changes its endpoint. Growth consumes adjacent spans
   instead of overlapping them. Shift-drag inserts or removes lane time by moving
-  every later cel by the same delta.
+  every later clip by the same delta.
 - At a shared boundary, the left and right hit zones trim one side. The center
   is a rolling edit: right-edge resize followed by left-edge resize at one frame.
 - Split, trim-in, and trim-out are direct buttons and are disabled without an
@@ -156,9 +169,10 @@ The implemented correction slice and its remaining UI limits are recorded in
 
 ## 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.
+- **Audio remains outside visual lanes.** `symbol/lane-problems` deliberately
+  requires symbol instances. Audio is still an independent root node that can
+  link to picture; making audio itself lane-based would need an explicit lane
+  capability rather than a mixed child rule.
 - **`: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 —
@@ -172,9 +186,9 @@ The implemented correction slice and its remaining UI limits are recorded in
   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.
+- **Slip and retime are still absent.** The timeline action strip now applies
+  split/trim uniformly to a selected root or lane clip, but source-time slip and
+  retime still need their own proved semantics before they become controls.
 - **`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
@@ -186,14 +200,14 @@ The implemented correction slice and its remaining UI limits are recorded in
 
 From `frontend/`:
 
-    npx shadow-cljs compile test && node out/node-tests.js     # 437 tests, 5,804 assertions
+    npx shadow-cljs compile test && node out/node-tests.js     # 469 tests, 9,592 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
+    node --experimental-websocket test/browser/lane.mjs                  # generic symbol-lane 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