MBTiles selection: client-only rect select for vector tiles

[tmshv]

Summary

Fixes #214. MBTiles vector tile rect select now works via client-only spatial queries, with a GeoJSON overlay handling highlighting.

Previously, rect select had no effect on MBTiles sources because MVT features from `queryRenderedFeatures` lack numeric IDs when the PBF tiles don't encode them — the serialization step dropped every feature, and `setFeatureState` couldn't match anything.

Approach

• `queryRenderedFeatures` is the sole spatial query for MVT sources (no backend spatial query)
• New `assignFeatureIds` helper assigns synthetic sequential IDs to features without PBF IDs
• New `MvtSelectionOverlay` renders selected features as a GeoJSON overlay (replaces `setFeatureState` for MVT sources)
• Only solid rect (contains) mode for MVT — no intersect mode

Limitations

• Selection limited to currently rendered features at the active zoom
• Zoom changes after selection: highlights persist (stored as GeoJSON), but new features aren't auto-selected