The timeline opens the whole document

Making a lane one row cost the thing a row was for. A clip stopped being a row,
so there was no longer any way to open a clip and see what was inside it, and
the inside of a drawing — the most ordinary thing in the document — became
reachable only by opening it as its own tab. This is that capability back,
from the root timeline, down as far as it goes.

An expanded lane opens exactly ONE clip: the selected one. Its own keys, then
the lanes and nodes of the symbol it places, then theirs, each mapped into this
ruler by the recursive walk that was already there. Twelve clips in a lane
still cost one row, and inspection costs one branch rather than twelve.

Two things that only showed up once it ran. The portal is chosen by the whole
LINEAGE of the selection and not by the selected id: selecting a shape inside
the clip — or the end of its span — is still working inside that clip, and
matching the id alone shut the portal the instant anything under it was
touched. And selecting now waits for the pointer to come UP, because selecting
on the way down re-drew the timeline before the gesture had said anything: it
shut the portal holding the lane being dragged INTO, out from under the
pointer.

A HELD clip opens too, which the old row walk never did either. `source-time`
is nil for a hold, so the walk stopped there and the contents of every drawing
were invisible from here. Its rows are shown across the hold — which is when
the node is on screen — and marked `:unmapped?`: no keys, and no draggable
edges, because a frozen clock gives no frame inside it a place on this ruler.
Refusing to place the keys is the honest half; refusing to show the rows was
not.

Double-clicking a clip opens the symbol it places as a tab, as double-clicking
the same symbol in the pool does. That was already written and had never once
run: the track captures the pointer for a slide, so the click and double-click
that follow are delivered to the track and never to the block. The track now
resolves them itself. Fixing the delivery exposed two more: `symbol/lineage`
reported a `parent cycle` for any id in a symbol with NO nodes, because a
one-element chain is longer than zero nodes — and opening a symbol left the
selection pointing into the symbol being left, which the breadcrumb and the
inspector then tried to resolve. The editor unmounted. Both are fixed where
they were wrong, and the browser test asserts the editor is still standing
afterwards.

Audio is a clip in a lane like everything else. A dropped sound lands in one
and is trimmed and moved by the same commands; a lane holds picture or sound
and not both, which is the explicit capability the model asked for rather than
a guess per frame. The refusal lives in the commands and not only in
validation, because placement claims time: `blank` would have deleted the
sound to make room for the picture and left a perfectly valid document behind.
What is in a lane of the open symbol is drawn as a lane; what is nested inside
a placed symbol is still flattened by `audio-tracks`, so no sound is on two
rows.

Everything that enters the timeline now enters a lane: a converted take, a
symbol brought in from another project, a sound. One rule answers where —
`lane-destination` — and every symbol is born with a lane for it to answer
with. An unaimed drop fills an EMPTY lane rather than taking an occupied one
nobody pointed at, because the alternative is trimming away what was there to
make room for what was dropped.

Shift during a clip-body drag means the other intention: put this node INSIDE
the symbol the clip under the pointer places, through `nest/move-node`, which
is what keeps the world transform and the root timing. Overlap cannot say
which of the two is meant — dropping on occupied time already means claiming
it — so the person says, and a label by the pointer says it back. The label
asks `nest/move-refusal`, the same check the command makes, so it cannot
promise what the drop would refuse. Today it refuses more than it allows:
both clips have to be on screen at one frame, which two clips in one lane
never are, and a held destination has no clock to move through at all.
`docs/lane-nesting-notes.md` argues that the second refusal is stronger than
the facts require and says what would settle it.

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

docs/lane-handoff.md

@@ -86,15 +86,40 @@ decision, not a cleanup.
   `{: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 clip is not a row.** Rows, expansion and selection are editor state. The
-  document has never known about rows and must not learn.
+  document has never known about rows and must not learn — which is what let
+  the row model change three times in one sitting (blocks, then a
+  selected-clip portal, then sound lanes under the audio heading) without
+  touching a single document.
 - **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.
+- **Every symbol is born with a lane,** `clip/lane-node`, id `:lane`. A symbol
+  with none had nowhere to drop a thing, which made the first drop into any
+  symbol a special case. An unaimed drop fills an EMPTY lane that is already
+  there and otherwise makes a new one; it never takes an occupied lane nobody
+  pointed at, because placement claims time and would trim or delete what was
+  in it.
 - **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.
 
+## The timeline opens the whole document
+
+Expanding a lane opens exactly one clip — the selected one — and that portal
+opens the lanes and nodes of the symbol it places, recursively, mapped into
+the open symbol's ruler. The portal follows the LINEAGE of the selection, so
+working on something nested keeps the rows that revealed it open. A held clip
+opens too, with its rows marked `:unmapped?`: shown across the hold, with no
+keys and no draggable edges, because a frozen clock gives its frames no place
+on this ruler. `docs/lane-nesting-notes.md` has the reasoning and what is
+still missing.
+
+Double-clicking a clip opens the symbol it places as a tab, the same as
+double-clicking that symbol in the pool. Shift while dragging a clip body
+turns the temporal move into a structural one — see the nesting notes for why
+that is mostly refused today.
+
 ## Current timeline interaction
 
 - Creating a symbol inside an aimed lane creates a one-frame held clip at the
@@ -169,10 +194,16 @@ The implemented correction slice and its remaining UI limits are recorded in
 
 ## Known gaps and traps
 
-- **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.
+- **Audio is a clip in a lane too, and a lane holds one kind.** A sound placed
+  from the pool lands in a lane and is moved and trimmed by the same commands
+  as picture. The capability the earlier note asked for is the homogeneity
+  rule rather than a field: `symbol/lane-problems` refuses a lane holding both
+  kinds, and `lane/place-symbol` and `lane/adopt` refuse BEFORE claiming time,
+  because placement claims time and would otherwise have deleted the sound to
+  make room for the picture and left a valid document behind. Audio nested
+  inside a placed symbol — a take's own sound — is still shown flattened by
+  `nest/audio-tracks`; what is in a lane of the open symbol is drawn as a lane
+  and not flattened twice.
 - **`: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 —