One word for one thing: it is a cel

Four words had accumulated for a node that puts a symbol inside another symbol.
`instance` was the document's, from the model. `placement` was the stage and
export work's. `occurrence` came in with the lane model. `exposure` came in with
me, because it is what an animator would say. Three bodies of work each brought
a word and none of them retired anybody else's, which is how you get a codebase
that reads like three people describing the same object over each other.

It is a CEL. One drawing, held for some duration. `cel` was already the view's
word — `.tl-cel`, the cel strip — so choosing it was also the smallest change,
and the app already says "drawing" for the content, which is what frees the word
up: historically a cel IS the celluloid with the drawing on it, and that sense
has somewhere else to live here.

  instance   the `:kind`. The general thing, anywhere in a document.
  cel        an instance in a lane. UI labels, command names, prose.
  lane       the group with `:layout :sequence`.
  drawing    the content a cel names.
  placement  kept ONLY for where a node sits — `nest/placement` and the
             transform that puts a face on the stage. Retired as a noun for the
             node itself.
  occurrence gone.

AND IT SETTLES A COLLISION I SHOULD HAVE SEEN EARLIER. `:time :expose` already
existed and means something else entirely: how many frames each step of a
subtree lasts, which is what shooting on twos is. Had the block been called an
exposure too, `node/expose`, `clock/exposed-frame` and `subs/render ::exposure`
would have been permanently confusable with it. Choosing `cel` lets the word
`exposure` keep the thing it actually names, and every remaining use of it in
`src` is now that one.

`:layout :sequence` stays as the field, and it is the one place two words are
kept deliberately: the layout names the RULE — children follow one another and
may not overlap — and a group carrying it is called a lane. `node/lane?` says so
where the two meet.

The second view is traditionally the exposure sheet. It will be the CEL SHEET,
for one vocabulary.

Renamed with a script and then read, because a blind pass does real damage: it
produced "an cel" thirty times, renamed the `::exposure` sub that is about the
`:expose` grid, and turned an "exposure grid" into a "cel grid" in two
docstrings. All three classes are fixed. `arthur.domain.sequence` is now
`arthur.domain.lane`, which is what its test file was already called.

424 tests, 5,749 assertions, and both browser flows — `test/browser/lane.mjs`,
renamed too.

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

docs/architecture.md

@@ -822,7 +822,7 @@ collaborator's keying. The fix is addressing, not an algorithm:
 
 ```
 palette
-sequence/:sid
+lane/:sid
 clip/:cid/timing                    clip rate
 clip/:cid/subject/:sid              tracked subject and settings
 clip/:cid/feature/:fid              tracked feature and settings