M3.15: Timeline tabs per open comp; Project double-click opens; tab drag insertion mark

- The Timeline shows one tab per open comp (dock DocTab). Clicking a tab shows that comp; ×
  or middle-click closes that comp's Timeline (comp.close), not the panel. Opening a comp brings
  its viewer and Timeline tab to the front.
- Project panel: a double-click on an item's name opens it (a comp gets its own tab) instead of
  renaming; Enter and the context menu rename.
- Tab drags: over a tab strip an insertion mark shows where the tab lands, and it lands there
  (Layout::insert_tab), which also reorders a group's own tabs; side zones are unchanged.
- Tests: ui_tabs (double-click, tab switch / close, drag reorder and cross-group insert; an
  ignored mid-drag snapshot); the Timeline close test follows the new behaviour.
- Docs: parity, control protocol.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -85,7 +85,9 @@ Points are logical window coordinates. A target is `{id}` (the element's centre;
 | `ui.screenshot` | `{path?, panel?, id?}` | `{path, width, height}`: a PNG of the window, or cropped to one panel or element. |
 
 Element ids are stable, for example `tools.Selection`, `panel.Timeline`, `panel.tab.EffectControls`,
-`header.workspace.Animation`, `project.item.<id>`, `viewer.comp`, `viewer.handle.<layer>.<i>`,
+`panel.tab.Timeline.<comp>` (one Timeline tab per open comp; `panel.tab.Timeline` is the shown
+one), `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,
 Enter or a double-click renames, drag reorders), `timeline.prop.<uid>.stopwatch`,
 `timeline.key.<uid>.<frame>`, `timeline.cti`, `effectControls.prop.<uid>.value` and