M13.33: nested comp markers on the precomp layer bar

- commands::markers::nested_markers: a precomp layer's comp markers at their times in the
  containing comp (start and stretch in closed form; with time remapping or a stretch over
  protected regions, the first frame whose source time reaches them), within In–Out.
- markers.nested {layer}: the same list for agents.
- Timeline: drawn on the precomp bar as outlined read-only markers (positions cached per
  revision), hover names them, double-click opens the nested comp at the marker; automation id
  timeline.layer.<id>.nestedMarker.<i>.
- Tests: engine (start, stretch, trim, time remap, non-precomp error), UI (draw, label, open).
  Docs: parity, control protocol.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -89,7 +89,7 @@ Element ids are stable, for example `tools.Selection`, `panel.Timeline`, `panel.
 one), `panel.tab.Viewer(<n>)` and `viewers.<n>` (View ▸ New Viewer's other Composition viewers;
 `viewers.0` is the Composition panel while another viewer is active), `header.workspace.Animation`, `project.item.<id>` (a double-click opens the item, Enter
 renames it), `viewer.comp`, `viewer.handle.<layer>.<i>`,
-`timeline.layer.<id>.bar`, `timeline.layer.<id>.twirl`, `timeline.layer.<id>.row` (click selects,
+`timeline.layer.<id>.bar`, `timeline.layer.<id>.nestedMarker.<i>` (a precomp's comp markers), `timeline.layer.<id>.twirl`, `timeline.layer.<id>.row` (click selects,
 Enter or a double-click renames, drag reorders), `timeline.prop.<uid>.stopwatch`,
 `timeline.key.<uid>.<frame>`, `timeline.cti`, `effectControls.prop.<uid>.value` and
 `effects.item.<name>`. The Composition viewer adds `viewer.magnification`, `viewer.resolution`,

docs/parity.md

@@ -422,6 +422,12 @@ layers); the Graph Editor has Auto-Select Graph Type (the default: the speed gra
 spatial properties are shown, else the value graph; choosing Value or Speed turns it off). Graph
 Editor keys are small squares whatever their interpolation, as in After Effects.
 
+Since (M13.33): a precomp layer's bar shows its comp's markers, as outlined, read-only markers.
+They are mapped through the layer's timing (start, stretch, time remapping, Responsive Design
+stretch) and shown within its In–Out range. Hovering names them ("beat (marker in Pre)"), and a
+double-click opens the nested comp at the marker. Agents read the same list with
+`markers.nested {layer}`.
+
 Since (M13.32): View ▸ New Viewer (Alt+Shift+N, also in the Composition panel menu) opens
 another Composition viewer as its own panel, as in After Effects. The viewer in use is locked,
 so it keeps its comp. One viewer is active: it is the interactive one and shows the active comp.
@@ -751,8 +757,8 @@ are pinned bit for bit by golden hashes (`crates/effects/tests/particle_golden.r
     animators).~~ (M4.13)
 17. ~~Content-keyed RAM preview frames, so an edit keeps the frames of comps it doesn't touch.~~
     (M4.14)
-18. ~~View ▸ New Viewer (several Composition viewers)~~ (M13.32); nested comp markers on the
-    precomp layer bar, audio scrubbing.
+18. ~~View ▸ New Viewer (several Composition viewers)~~ (M13.32); ~~nested comp markers on the
+    precomp layer bar~~ (M13.33), audio scrubbing.
 
 At the original audit the engine underneath (keyframes, expressions, shape operators, text
 animators, Classic 3D, all 38 blend modes, the render queue) was already deep, and most of what was