[M4] Comparison mode: pin TextFrame to a specific document

[Korede-TA]

Goal

Allow two TextFrames to be pinned to specific documents for side-by-side comparison. Selecting a chunk in any visualization frame highlights it in both panels.

What to do

• Add pinnedDocId: NodeId | null to per-pane config in view-store.frameConfigs
• TextFrame: when pinnedDocId is set, ignore global focused node and instead scope to chunks of that document
  • Show document title at top with a "📌 pinned" indicator + unpin button
  • Chunk list is scrollable; currently focused chunk is highlighted in place
• "Pin to document" button in TextFrame toolbar (FrameContainer menu): opens document picker dropdown
• Default comparison layout preset: { direction: 'row', first: 'text', second: 'text' } with each pane pinned to a different doc
• Add comparison to layout presets in layout-store

ScrollTo integration (depends on #41)

• When selection changes and the newly focused node belongs to the pinned document, scrollTo that chunk within the panel

Acceptance criteria

• Two TextFrames can be open simultaneously, each pinned to a different document
• Selecting a chunk in SemanticFrame highlights it in the correct pinned panel
• Unpin button restores global focus mode