Keep map pins locked to the map while panning

[dfalling]

Summary

Map pins visibly trailed behind the map during a pan, as if their layer refreshed slower than the map's.

The cause is that <Marker> isn't part of the map on Android — it's an Android View parented to the MapView and repositioned by hand once per rendered frame (MLRNMapView.kt:713 → MarkerViewManager.updateMarkers()). The map renders into a GLSurfaceView, so its content is composited independently of the view hierarchy holding the pins, and the pins land a frame or more behind the map content they're pinned to. That happens with one pin; a viewport full of them just makes it obvious, since the per-frame work is O(pins). The library's own docs point at a symbol layer for this reason.

Clustering would only have reduced the pin count without touching the desync, so it wasn't the right fix.

Pins are now a <Layer type="symbol"> over a <GeoJSONSource>. Symbols render inside the map's own GL frame, so they're pixel-locked to it and the count barely matters.

• Each distinct element emoji is rasterised off-screen once (react-native-view-shot) and registered as a map image. Emoji can't come from the style's glyphs — those are single-channel SDF and OpenFreeMap's Noto Sans set has no emoji ranges — so an image is the only route.
• symbol-sort-key replaces the manual array reordering that floated the selected marker to the end.
• icon-size replaces the grow-on-select transform.
• The client-side viewport cull is gone; MapLibre culls per tile, which also stops pins popping in at the screen edges.
• Because icon offsets scale with icon size, a selected pin's tip now stays exactly on its coordinate instead of dipping ~2pt below it.

Test plan

• tsc --noEmit, biome check, and all 28 tests pass.
• Built and ran on an emulator. Verified pins draw as the correct red teardrops with the emoji upright, the no-icon fallback pin renders, the selected pin is larger and drawn above its neighbours, the drop shadow survives the off-screen capture uncropped, and tapping a pin reports the right element id. Checked in both light and dark themes.
• Not verified: that the judder is actually gone at real pin counts — that needs a real device with a real dataset, which the emulator and an unauthenticated session can't provide. Worth a look before merging.

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