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>

frontend/test/browser/lane.mjs

@@ -59,7 +59,7 @@ try {
   await evaluate(`(() => {
     const k = cljs.core.keyword;
     cljs.core.swap_BANG_(re_frame.db.app_db, db => cljs.core.assoc(db, k('route'), k('local-test')));
-    window.sequenceSnapshot = () => {
+    window.laneSnapshot = () => {
       const db = cljs.core.deref(re_frame.db.app_db);
       const entry = arthur.footage.store.entry(cljs.core.get(db, k('clip/current')));
       return cljs.core.clj__GT_js(entry);
@@ -75,7 +75,7 @@ try {
     })()`), `enabled button: ${label}`);
     await sleep(180);
   };
-  const shot = async () => (await evaluate('sequenceSnapshot()'));
+  const shot = async () => (await evaluate('laneSnapshot()'));
   const instances = s => Object.values(s.clip.symbols.main.nodes).filter(n => n.kind === 'instance')
     .sort((a, b) => a.time.at - b.time.at);
   await click('+ lane');
@@ -90,7 +90,7 @@ try {
   assert.equal(await evaluate('document.querySelectorAll(".tl-label:not(.tl-corner)").length'), 1);
   assert.equal(await evaluate(`cljs.core.get_in(cljs.core.deref(re_frame.db.app_db),
     cljs.core.vector(cljs.core.keyword('playback'), cljs.core.keyword('frame')))`), 4,
-    'new drawing seeks to its occurrence');
+    'new drawing seeks to its cel');
 
   // Shorten this test shot to the occupied extent, purely in memory.
   await evaluate(`(() => {
@@ -112,9 +112,9 @@ try {
   assert.equal(s.history.done.length, before.history.done.length + 1);
   await evaluate(`document.dispatchEvent(new KeyboardEvent('keydown', {key:'z', ctrlKey:true, bubbles:true}))`);
   await sleep(250);
-  assert.deepEqual((await shot()).clip, before.clip, 'one undo restores exposure, ripple, and shot length');
+  assert.deepEqual((await shot()).clip, before.clip, 'one undo restores cel, ripple, and shot length');
 
-  // Sharing: one drawing exposed twice, then one exposure decoupled. Room is
+  // Sharing: one drawing exposed twice, then one cel decoupled. Room is
   // made first so these assertions are about content and not about overflow.
   await evaluate(`(() => {
     const k = cljs.core.keyword, db = cljs.core.deref(re_frame.db.app_db);
@@ -135,19 +135,19 @@ try {
   assert.equal(cels[2].source.symbol, cels[0].source.symbol, 'reuse exposes the same drawing');
   assert.equal(await evaluate('document.querySelectorAll(".tl-cel").length'), 3);
   assert.equal(await evaluate('document.querySelectorAll(".tl-label:not(.tl-corner)").length'), 1,
-    'three exposures, still one row');
+    'three cels, still one row');
   assert.equal(await enabled('make unique'), true);
   await click('make unique');
   s = await shot();
   cels = instances(s);
-  assert.notEqual(cels[2].source.symbol, cels[0].source.symbol, 'that exposure has its own drawing');
+  assert.notEqual(cels[2].source.symbol, cels[0].source.symbol, 'that cel has its own drawing');
   assert.equal(await enabled('make unique'), false, 'and is not shared any more');
   await click('duplicate');
   s = await shot();
   cels = instances(s);
   assert.equal(cels.length, 4);
   assert.equal(new Set(cels.map(n => n.source.symbol)).size, 4,
-    'four exposures of four drawings: nothing is shared once every copy is made');
+    'four cels of four drawings: nothing is shared once every copy is made');
   assert.equal(s.history.done.length, before.history.done.length + 3, 'three more commands, three more steps');
 
   // A drawing into the middle of a hold: split, then insert. Both act at the
@@ -158,25 +158,25 @@ try {
   assert.deepEqual(placed(s), [[0, 4], [4, 5], [5, 6], [6, 7]]);
   await evaluate(`document.querySelector('.tl-cel').click()`);
   await sleep(200);
-  assert.equal(await enabled('split'), false, 'the start of an exposure is not inside it');
+  assert.equal(await enabled('split'), false, 'the start of a cel is not inside it');
   await click('+1');
   await click('+1');
   assert.equal(await enabled('split'), true);
   await click('split');
   s = await shot();
   assert.deepEqual(placed(s), [[0, 2], [2, 4], [4, 5], [5, 6], [6, 7]],
-    'one exposure became two, over the frames it had');
+    'one cel became two, over the frames it had');
   await click('insert');
   s = await shot();
   assert.deepEqual(placed(s), [[0, 2], [2, 3], [3, 5], [5, 6], [6, 7], [7, 8]],
     'the new drawing took frame 2 and everything from there rippled later');
   assert.equal(await evaluate('document.querySelectorAll(".tl-cel").length'), 6);
   assert.equal(await evaluate('document.querySelectorAll(".tl-label:not(.tl-corner)").length'), 1,
-    'six exposures, still one row');
+    'six cels, still one row');
   assert.equal(s.history.done.length, before.history.done.length + 5);
 
   // Trim, move and blank: three gestures that move nothing but their own
-  // exposure, and a shot whose length does not follow what is in it.
+  // cel, and a shot whose length does not follow what is in it.
   await evaluate(`[...document.querySelectorAll('.tl-cel')][2].click()`);
   await sleep(200);
   assert.deepEqual(placed(await shot()).slice(2, 4), [[3, 5], [5, 6]]);
@@ -185,7 +185,7 @@ try {
   await click('trim out');
   s = await shot();
   assert.deepEqual(placed(s), [[0, 2], [2, 3], [3, 4], [5, 6], [6, 7], [7, 8]],
-    'it ends at the playhead and every other exposure stayed');
+    'it ends at the playhead and every other cel stayed');
   assert.equal(await enabled('move here'), true);
   await click('move here');
   s = await shot();