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-model.md

@@ -1,6 +1,6 @@
 # The Lane Model
 
-Revised 2026-10-01. Cel ownership, source playback, one-row drawing lanes,
+Revised 2026-10-01. Clip ownership, source playback, one-row generic lanes,
 direct clip movement and edge editing, correction evaluation, and correction
 authoring for rotation and position are implemented. The former cel-sheet
 projection was removed: the timeline is the single timing interface. Sections
@@ -147,8 +147,9 @@ cel transform and then the content's own transform.
 The interval in lane time is derived through `:time`; do not also store parent
 start/end values. Sequence children require finite intervals and positive
 placement rates. Ordering and overlap checks use the mapped intervals, not `:z`.
-The sequence group may contain visual cels or audio cels; its
-capability must reject an incompatible mixture rather than infer it per frame.
+The current sequence group contains visual symbol clips. Audio remains an
+independent root node (and can be linked to picture); if audio lanes are added,
+their capability must be explicit rather than inferred per frame.
 
 A source reference is fixed within a cel. The lane changes content when
 another cel becomes active. This is a deliberate revision of the original