A held cel is a one-frame symbol that loops

Holding a drawing and looping a one-frame symbol are the same picture, and the
second is a case the model already carries, so the first is a special case
kept for nothing. Speed 0 goes; a cel becomes a one-frame symbol with
`:end :loop` and a span. Looping and span are already properties of the
instance and `:frames` already belongs to the symbol, so nothing moves — a
mode is deleted. Two spellings of looping collapse to one, `:time :loop?`
giving way to `:playback :end :loop`, and `extend-hold` collapses into
`resize-out`, since it exists only to refuse anything that is not frozen
before editing a span.

What it buys is one rule where there were three refusals. `nest/inside` has no
invertible clock for a hold, for `:end :hold`, or for a loop, so
shift-to-reparent refuses all three — which is most of what anybody would drag
onto. Resolving the move with the destination's map at the CURRENT frame
covers every one: a loop is affine within the period the frame falls in, a
one-frame loop is that rule with a period of one — which lands exactly where
the nesting notes argued it should from first principles — and `:end :hold` is
affine in the played part and frozen in the tail.

The trap is written down beside it, because it would be found the hard way:
`audio-tracks` expands a loop into one walk per period, and is saved today
only by the `(pos? speed)` guard that a held cel fails. Make every drawing a
loop and a drawing held for 120 frames becomes 120 walks emitting any nested
sound 120 times. An audibility precheck has to land in the same change.

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

docs/lane-is-a-view-plan.md

@@ -123,6 +123,82 @@ people, and it does not make the symbol a different kind of thing. The thing
 to hold the line on is that nothing outside the timeline, and the commit
 path's overlap check, is allowed to read it.
 
+## Tear out the held cel
+
+A held cel is a 1-frame symbol shown for many frames. A 1-frame symbol with
+`:end :loop`, lengthened, is the same picture by a different route — and the
+second route is a case the model already has, so keeping the first one is
+keeping a special case for free.
+
+**What is already true**, so that nothing has to move: `:span` is on the node,
+in its own frames; `:time` (`:at`, `:rate`) is on the node; looping is on the
+node. The SYMBOL owns only `:frames`, the authored window. So looping and span
+are instance properties already, and this change is about deleting a mode, not
+relocating a field.
+
+**What has to be decided and collapsed:**
+
+- There are two spellings of looping — `:time :loop?` and `:playback :end
+  :loop` — and both are read, in `node/placed-frame` and in
+  `nest/audio-tracks`. Keep one. `:playback {:in :speed :end}` already says
+  what happens at the ends, so `:end :loop` is the one to keep and
+  `:time :loop?` is the one to delete.
+- `:playback :speed 0` stops being produced. Make it illegal in
+  `node/problems` rather than legal-but-unused, so a frozen clock has exactly
+  one spelling: a 1-frame loop.
+- `lane/extend-hold` exists only because holds were special — it refuses
+  anything whose speed is not 0 and then edits a span. Once a hold is a loop,
+  lengthening one IS `resize-out`, and the command collapses into it.
+- `cel` survives as the word for the creation policy — a new empty symbol is
+  one frame — but stops naming a playback mode.
+
+**What it buys, and this is the point:** one rule for nesting, which settles
+the refusal that blocks shift-to-reparent today. `nest/inside` currently has
+no `:time` for a hold, for `:end :hold`, or for a loop, and so refuses all
+three. The general rule that covers all of them: **resolve the move with the
+destination's map at the CURRENT frame — the affine piece the current frame
+falls in.**
+
+- A loop of length L is affine within the period the current frame is in.
+  Timing is preserved inside that period and repeats after it, which is what
+  looping means.
+- A 1-frame loop — the ex-hold — has a period of length 1, so the map within
+  it is trivially invertible and lands the moved node on frame 0, aligned to
+  the current frame. That is exactly the answer `docs/lane-nesting-notes.md`
+  argues for from first principles, arrived at here as an instance of the
+  general rule instead of a special case.
+- `:end :hold` is affine in the played part and frozen in the tail, which the
+  same sentence covers.
+
+**The trap, which must be handled in the same change.** `nest/audio-tracks`
+expands a loop into one walk PER PERIOD:
+
+    periods (range (floor (/ (to-local source lo) length))
+                   (ceil  (/ (to-local source hi) length)))
+
+Today a held cel is skipped entirely — `(pos? speed)` is the guard, and the
+comment says a visual freeze does not emit a sustained audio sample. Turn
+every drawing into a 1-frame loop and that guard stops firing: a drawing held
+for 120 frames becomes 120 recursive walks, and any sound inside it is emitted
+120 times. That is both a wrong mix and a performance cliff on the most common
+node in the document. Required with this change: a cheap `symbol/audible?`
+precheck so a source with no audio anywhere inside it is never period-expanded,
+and a cap or a different formulation for the ones that are.
+
+Also worth knowing before the change: `ui/timeline`'s own docstring already
+says a looping instance draws only its first pass. With every drawing a loop,
+that sentence now describes every drawing — harmless, since one pass of a
+1-frame symbol is the whole of it, but the docstring should stop sounding like
+a limitation.
+
+**Dropping into a 1-frame symbol.** The window is authored and crops what it
+holds, so a 10-frame symbol dropped into a 1-frame drawing shows its frame 0
+and nothing else. That is consistent — `:frames` is the shot length and
+`:extent :grow-symbol` is the opt-in — but it is probably not what somebody
+dragging means. Offer the growth through the `:required-frames` retry the
+model already uses: the command reports what it would need, the UI offers one
+button.
+
 ## Order of work
 
 Each step compiles, passes `npm test`, and leaves the editor usable.
@@ -183,7 +259,8 @@ to keep.
 2. **Does a lane row's edge drag trim the placing instance's span, or ripple
    the children?** Same handle, two commands; the row is now an instance, so
    it has a span of its own for the first time.
-3. **The held destination for shift-to-reparent** is still refused —
+3. **Does tearing out the held cel come before or after the lane work?** It
-   `nest/inside` has no invertible clock for a hold, which is most of what
+   is independent of it — `nest` never knew about lanes — and it is what makes
-   anybody would try to nest into. `docs/lane-nesting-notes.md` argues the
+   shift-to-reparent work on the thing people would actually drag onto. Doing
-   refusal is stronger than the facts require and says what would settle it.
+   it first means the lane work lands on a model with one playback mode fewer;
+   doing it after means two changes to `lane_test`'s fixtures instead of one.