Add public dimmedNodeIds and highlightedNodeIds props with union merge and fix NodeRenderer fade

[fayekelmith]

Summary

• Adds two new render-only props to <SystemCanvas>: dimmedNodeIds?: Set<string> and highlightedNodeIds?: Set<string>, driven by the host application's own state (external search, filter, or agent-driven focus).
• Both props union-merge with the internal Cmd+F search-derived sets, so both sources can be active simultaneously and search keeps working unchanged. Merging is handled by a module-local unionIdSets helper (exported for unit tests only).
• Emphasis is strictly render-only: dimming never moves nodes, affects layout/auto-fit, or changes selection/drag/edit behaviour, but propagates visually to connected edges (dotted, faded) to match internal search behaviour. A node in both sets renders both visuals (ring nested inside the dimmed group, inheriting its opacity).
• Fixes a NodeRenderer bug: the mount fade-in left inline style.opacity/transition on the node <g> permanently, which shadowed the SVG presentation attribute and froze dim/undim flips. Inline styles are now cleared after the mount fade (with timer cleanup on exit/unmount), mirrored after Viewport's triggerFade pattern.
• Adds the first full SystemCanvas mount test suite (SystemCanvas.emphasis.test.tsx, 20 tests) with an inline jsdom harness (ResizeObserver + SVGSVGElement stubs).

Testing

• npm run typecheck — clean
• cd packages/react && npx vitest run src/components/__tests__/SystemCanvas.emphasis.test.tsx — 20/20 passing