[M2] deriveLabel utility + NodeTooltip component (all frames)

[Korede-TA]

Goal

Replace raw node IDs in tooltips and breadcrumbs with meaningful labels. Single shared tooltip component across all deck.gl frames.

src/lib/derive-label.ts

Priority chain:

1. node.properties.title (categorical value)
2. node.properties.source (categorical — filename/URL)
3. First 80 chars of node.text truncated at word boundary + "…"
4. Fallback: node.id

src/components/NodeTooltip.tsx

• Rendered as a React portal at document.body
• Positioned near cursor via pointerX/Y state (tracked in a thin mouse-move listener)
• Triggered by selectionStore.hovered — visible when any node is hovered
• Shows:
  • Type badge (colored with NodeType.display.color)
  • Derived label (bold)
  • Text preview (2 lines max, truncated)
  • Annotation count chips: 🌍 n geo · 📅 n temporal · 👤 n entity
• Dismisses when hovered is null
• z-index above all frames

Integration

• Mount <NodeTooltip /> once in AppShell
• All deck.gl frames already dispatch hover to selectionStore — no per-frame changes needed
• Breadcrumb ([M4] Breadcrumb nav + browser history (level/focus changes) #25) and TextFrame node nav use deriveLabel instead of raw IDs

Acceptance criteria

• Hovering any node in any frame shows tooltip within 100ms
• Tooltip shows meaningful text, not raw IDs
• Disappears cleanly when cursor leaves a node