The shot is as long as somebody said it was

Trim, move and blank, and the decision they all three walked into: is the shot's
length authored, or derived from what is in it?

AUTHORED. `:frames` is the symbol's window — how long the shot IS — and the
occupied extent of its lanes is a different fact, read off the occurrences. A
command grows the window when the caller says `:grow-symbol` and NEVER shrinks
it, so blanking the end of a shot leaves a shot with empty frames at the end.
That is a true statement about what somebody authored, and the alternative is
deleting the last drawing and quietly shortening the film. `finish` had the
right behaviour by accident — `(apply max (:frames sym) ...)` — and now says
which number is which: `needed` is where the occurrences reach, `:frames` is
what was authored, and the only thing that makes the second follow the first is
a caller asking.

The three commands turned out to be one piece of geometry, which is `split`'s.
A `:span` is in the occurrence's OWN frames and `:time` says where those land in
the lane, so moving an edge of an exposure is ONE WRITE to `:span` and `:time`
and `:playback` are never touched. `local` and `edged` are the whole of it, and
split now goes through them too.

  trim   narrows one edge and moves nothing else. Lengthening is `extend-hold`,
         which carries a ripple policy and a shot-length policy because it needs
         them; letting trim grow as well would give one gesture two sets of
         rules and a way to overlap its neighbour.
  move   one write to `:time :at`, and a destination that would overlap is
         REFUSED rather than rippled. Moving a drawing and re-timing the ones
         around it are different intentions, and a move that pushed the rest
         would be the second wearing the first one's name. Clear the room first.
  blank  leaves a gap and does not close it. Wholly inside the range goes,
         overlapping an end is trimmed to it, spanning the range is split — the
         one case that needs an ID, and it asks for one instead of inventing it.

Because the source clock is untouched, trimming the front of a playing insert
starts it LATER INTO its animation rather than restarting it, which is the
difference between trimming and slipping and the reason they stay two commands.
The test samples the frames it kept and asserts they show what they showed.

Blanking leaves the drawings in the library. A lane does not own its content,
and a drawing whose last exposure is gone is still a drawing somebody made.

Overwrite is now `blank` then `place` and needs no policy argument of its own,
which is why it still is not one.

424 tests, 5,749 assertions. The browser flow trims an exposure at the playhead,
moves it into the gap that made, blanks it, and checks the shot is still as long
as it was authored.

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

frontend/test/browser/sequence.mjs

@@ -174,8 +174,31 @@ try {
   assert.equal(await evaluate('document.querySelectorAll(".tl-label:not(.tl-corner)").length'), 1,
     'six exposures, 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.
+  await evaluate(`[...document.querySelectorAll('.tl-cel')][2].click()`);
+  await sleep(200);
+  assert.deepEqual(placed(await shot()).slice(2, 4), [[3, 5], [5, 6]]);
+  await click('+1');
+  assert.equal(await enabled('trim out'), true, 'the playhead is inside it');
+  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');
+  assert.equal(await enabled('move here'), true);
+  await click('move here');
+  s = await shot();
+  assert.deepEqual(placed(s), [[0, 2], [2, 3], [4, 5], [5, 6], [6, 7], [7, 8]],
+    'and moves to the playhead, into the gap it just made');
+  await click('blank');
+  s = await shot();
+  assert.deepEqual(placed(s), [[0, 2], [2, 3], [5, 6], [6, 7], [7, 8]],
+    'blanked: a gap where it was, and nothing closed it');
+  assert.equal(s.clip.symbols.main.frames, 20, 'the shot is as long as it was authored');
+  assert.equal(s.history.done.length, before.history.done.length + 8);
   assert.equal(errors.length, 0, JSON.stringify(errors));
-  console.log('PASS: create lane/drawings, one-row cels, hold ripple, seek, explicit overflow, atomic undo, reuse/make unique/duplicate, split and insert; no server writes');
+  console.log('PASS: create lane/drawings, one-row cels, hold ripple, seek, explicit overflow, atomic undo, reuse/make unique/duplicate, split, insert, trim, move and blank; no server writes');
 } finally {
   if (ws?.readyState === WebSocket.OPEN) {
     ws.send(JSON.stringify({ id: 999999, method: 'Browser.close' }));