Fix canvas render lag: LOD fan-out, unmemoized markdown, prop churn

[dovvnloading]

Problem

The canvas UI was reported as glitchy/laggy, with nodes visibly fidgeting behind the cursor while dragging. Live measurement on a 63-node scene traced the dominant cost to useLodVisibility: it subscribed to the raw zoom number from React Flow's store rather than the derived LOD boolean, so every node view (~14 kinds) re-rendered on every animation frame of any zoom/pan gesture - not just when the LOD threshold actually crossed. That fan-out repeatedly re-ran NodeMarkdown's full remark/rehype/KaTeX/highlight.js parse pipeline, which was itself unmemoized. Measured cost: ~15ms of main-thread work per wheel step (near the full 16.6ms 60fps budget) on the same scene.

Secondary contributors: SceneCanvas's static <ReactFlow> props (deleteKeyCode, proOptions, defaultEdgeOptions, snapGrid) and event-handler props (onSelectionChange, onEdgeMouseEnter, onEdgeMouseLeave) were fresh literals/closures on every render; the drag-frame smart-guide state update fired unconditionally even when nothing changed; and the controlled-drag round-trip lag let the cursor drift outside a dragged node's true rect between frames, toggling :hover on/off and replaying its border/box-shadow transition as visible flicker.

Change

• useLodVisibility.ts - selector now returns the boolean directly (s.transform[2] < LOD_ZOOM_THRESHOLD) so subscribers only re-render on an actual LOD flip.
• NodeMarkdown.tsx - memoized the component; hoisted the static plugin/component config to module scope.
• SceneCanvas.tsx - hoisted static <ReactFlow> props to stable references, wrapped selection/edge-hover handlers in useCallback, made the smart-guide state update bail out via Object.is when unchanged.
• styles.css - pins node-card hover chrome to its resting state while .react-flow__node.dragging, so the round-trip lag can't trigger transition flicker.
• ChatNodeView.test.tsx - NodeMarkdown is now memo(...), an object rather than a directly-callable function, which broke the existing vi.fn(actual.NodeMarkdown) mock (.apply is not a function). Fixed by spying on a plain wrapper that renders the real memoized component via createElement, preserving both the original call-count assertions and genuine markdown rendering in tests.

Test plan

• npm run check passes end-to-end (schema, typecheck, lint - 0 errors, 1922/1922 tests, build, bundle-size)
• Live-measured before/after on the same 63-node scene: zoom cost ~15ms median (max 17.7ms) → ~1.4ms median (max 2.4ms) per wheel step
• Confirmed in-browser: drag/pan/zoom still function correctly, the new hover-suppression rule is live and reachable during an active drag, zero console errors on a fresh load