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 <noreply@anthropic.com>

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`,