feat: add MapLibre GL + PMTiles basemap support (dual-mode CRS)

[fank]

Summary

• Adds optional MapLibre GL basemap rendering under Leaflet with PMTiles protocol support
• Implements dual-mode CRS: legacy L.CRS.OCAP (pixel-based) for existing raster tiles, and L.CRS.EPSG3857 (geographic) for MapLibre basemaps
• Activated per-world by adding maplibreStyle URL to a map's map.json — no existing maps are affected

Changes

File	Changes
static/index.html	Added MapLibre GL v4.7.1, leaflet-maplibre-gl v0.0.22, pmtiles v4.4.0 (CDN)
static/scripts/ocap.js	Dual-mode CRS, armaToLatLng(), getMapImageBounds(), MapLibre layer, raster tile gating
static/leaflet/L.Layer.Grid.js	Grid coordinate conversion + zoom thresholds for EPSG:3857

How it works

• armaToLatLng() is the single gate for all coordinate conversions (20+ call sites, 5 files). Making it dual-mode means all entity, marker, event, and grid code works in both CRS modes without changes.
• Arma meters map to degrees at the equator via METERS_PER_DEGREE = 111320. Mercator distortion is negligible for sub-degree map extents (~0.1% error). No proj4 dependency needed.
• PMTiles protocol registered idempotently so style.json can use pmtiles:// URLs.

How to activate for a map

Add maplibreStyle to the world's map.json:

{
  "maplibreStyle": "https://maps.ocap2.com/styles/altis/style.json"
}

Remaining infrastructure (not in this PR)

• Create PMTiles from existing raster tiles
• Create style.json files per world
• Host pmtiles + style.json on maps.ocap2.com (needs HTTP range request support)
• Browser testing with actual MapLibre basemaps

Test plan

• Verify existing raster tile maps still work (no maplibreStyle = legacy mode)
• Verify grid overlay renders correctly in both modes
• Test with a maplibreStyle URL pointing to a valid style.json + PMTiles source
• Verify entity positions, markers, and events render correctly in MapLibre mode