feat: offline tile prefetch scheduler with LRU eviction and predictive caching

[real-venus]

Offline Geospatial Tile Prefetch Scheduler with LRU Eviction & Predictive Caching (#48)

Closes #48

What's included

File	Responsibility
src/types/tile.ts	TileId/TileMeta/GeoSample/Viewport/BBox/PrefetchRequest and bounds (2,500 cap, 2,250 evict threshold, 3×3 × ±2-zoom = 45-tile burst, >2 m/s prefetch, 30° stale-heading cancel, 7-day/48-h TTL by zoom, eviction check every 10 writes).
src/utils/tileMath.ts	Pure slippy-map math (lngLatToTile/tileBounds/tilesInBBox), trajectory prediction (predictCenter/burstTiles/predictBBox/zoomLevelsFor), heading/velocity gating (headingDelta/isStaleHeading/shouldPrefetch), and zoom-dependent TTL/isStale.
src/utils/lruEviction.ts	Doubly-linked LRU list; eviction prefers stale tiles first, then the lowest access_count / age ratio, with LRU as the final tie-break. Tracks size + byte usage.
src/services/tileCache.ts	IndexedDB tile-blob + metadata stores, in-memory LRU mirror for O(1) hit accounting, and an eviction pass triggered every 10 writes once the threshold is reached.
src/workers/tilePrefetch.worker.ts	Expands bbox × zoomLevels into tiles, fetches off the main thread, writes blobs to IndexedDB, skips already-cached tiles, and cancels superseded bursts (a sharp heading change supersedes the in-flight request).
src/hooks/useGeoLocation.ts	1 Hz navigator.geolocation watch → GeoSample (heading, speed).
src/hooks/useMapViewport.ts	Mapbox camera (center/zoom/bearing/pitch) as reactive state via a structural interface (no mapbox-gl dependency).
src/store/slices/tileCacheSlice.ts	Hit/miss/eviction/byte/pending stats + selectHitRatio.
src/components/dashboard/TileCacheOverlay.tsx	Feature-flagged debug overlay (hit ratio, fill %, bytes, evictions, pending).
tests/unit/*	Tile math (coords/bounds/prediction/TTL), LRU eviction ordering + staleness precedence, and the stats store.