feat: task 2.3 sprite preload (racing categories)

- src/map/assets/icons/: 9 placeholder SVGs (background plate, direction
  arrow, 7 categories: rally-car / quad / ssv / motorcycle / runner /
  hiker / default). Hand-authored simple silhouettes; replaced in
  Phase 3.8 with the branded set.
- src/map/core/categories.ts: SpriteCategory/SpriteColor types,
  mapCategoryToSprite() normaliser, inferColor() helper.
- src/map/core/sprite-preload.ts: idempotent preloadSprites() with
  memoised promise, whenSpritesReady() alias, getSpriteRegistry()
  read-only access, installSprites(map) for the mapReady flow.
  Composition pipeline draws the plate + composites a tinted icon
  centred on it (category sprites) or tints the arrow alone (direction
  sprites). Tint via canvas globalCompositeOperation: 'source-in'.
- src/main.tsx: void preloadSprites() fired at boot; the promise is
  memoised so mapReady flow awaits the same instance.
- src/map/core/map-view.tsx: onStyleData() awaits whenSpritesReady()
  AND _map.loaded() before installing sprites and flipping mapReady
  true. Sprites reinstall on every style swap.

Registry: 7 categories x 4 colours + 4 direction-only entries = 32
total. ~160KB in memory.

Deviations:
1. Direction sprites have no plate (it's a separate symbol layer in
   2.5 overlaid on the device sprite; double-plate would look wrong).
2. Hardcoded the design-system palette (#2E8C4A / #E8412B / #0E0E0C /
   #2563C8) directly. When 3.8 lands, these rebind to TRM tokens via
   CSS variables.

.planning/phase-2-live-map/README.md

@@ -50,7 +50,7 @@ When Phase 2 is done:
 | --- | ------------------------------------------------------------------------------------------ | ------ |
 | 2.1 | [MapView singleton + mapReady gate](./01-mapview-singleton.md)                             | 🟩     |
 | 2.2 | [Tile-source switcher](./02-tile-source-switcher.md)                                       | 🟩     |
-| 2.3 | [Sprite preload (racing categories)](./03-sprite-preload.md)                               | ⬜     |
+| 2.3 | [Sprite preload (racing categories)](./03-sprite-preload.md)                               | 🟩     |
 | 2.4 | [WS client + rAF coalescer + Zustand position store](./04-ws-client-and-position-store.md) | ⬜     |
 | 2.5 | [MapPositions (clustered + selected sources)](./05-map-positions.md)                       | ⬜     |
 | 2.6 | [MapTrails (bounded ring buffer, polyline rendering)](./06-map-trails.md)                  | ⬜     |