Precomp Phase G: timeline tabs, media bin section, stage preview #107

Phase G of the Precomps epic (#100, ADR-0029). Depends on Phase F (#106). Final v1 phase.

Scope

1. Timeline tabs: tab strip on the timeline panel host (src/ui/TimelineWindow.cpp wraps a single TimelineWidget); per-tab state = {Timeline*, selection}. TimelineWidget::setTimeline() exists (TimelineWidget.hpp:99) — audit cached per-timeline widget state for re-point safety; fallback = one widget instance per tab. Main-timeline tab is not closable. Sections/cues UI hidden in definition tabs (root-only concept).
2. Open paths: double-click a precomp instance (TimelineWidgetInput.cpp double-click handling) opens its definition's tab; OpenPrecompTabCommand{definitionId} for scripts/UI.
3. MediaBin precomp section: list definitions (name, canvas, duration, instance count); "New Precomp..." creation flow (name + canvas + fps + duration → CreatePrecompDefinitionCommand); drag source with payload PRECOMP_DEF carrying the definition id (pattern: MediaBinWindow.cpp:668-678); timeline accept branch alongside MEDIA_FILE/LAYER_KIND (TimelineWidgetRender.cpp:749-800) + drop callback wired in Engine (Engine.cpp:488-549 pattern) → CreatePrecompInstanceCommand. LayersWindow palette row for Kind::Precomp.
4. Stage preview: while a definition tab is active, Engine maintains one hidden preview instance (identity mapping against the tab Timeline's edit playhead, speed=1, no wrap, excluded from root UI and screen routing) flowing through the standard snapshot → PASS 1.6 path; stage view displays its compose target via the EditorReadGate-gated read (ADR-0014 D3D12Renderer m_composeTargets: show-thread emplace_back/resize races editor readers #75 /Video-texture slot free races editor ImGui frames holding the slot's SRV handle (same hazard class as #75) #89 protocol). De-scope fallback (pre-approved in ADR): if the gated read fights back, ship tabs-for-editing with preview via instances on the main timeline, and file the preview instance as a follow-up.
5. Compose soft-cap warning: editor-side toast/log when projected slot demand (screens + generatives + ×ばつeffect-layers + instances) ≥ 48 of 64.

Tests

• scripts/integration/precomp_tab_open.json: OpenPrecompTab → AddClipToPrecomp → close tab → assert instance resync (shadow count + render hash change on the main timeline).
• Preview: headless CaptureHash of the preview instance's RT if the gated read allows (stretch); otherwise a manual verification checklist posted on this issue before close.

Acceptance

• Author → drop → loop → edit-propagate workflow demonstrated end-to-end in the editor; local ctest green.
• Epic Epic: Precomps — nested timeline containers (ADR-0029) #100 acceptance items satisfied; flip ADR-0029 to Accepted (with amendments if implementation diverged).