Replace low-zoom fallback with custom global overview PMTiles

[Occumed79]

Problem

Production globe view still uses the OpenFreeMap fallback below zoom 6. The screenshot after PR #23 merged shows beige land, pink roads, crowded/clipped limb labels, and a visual result that does not match the intended luminous green-and-blue Occu-Med map.

This is caused by the current architecture:

• world-manifest.json publishes switchZoom: 6;
• the router selects only one regional archive based on the map center;
• below zoom 6 it retains the global fallback because one regional shard cannot cover the whole globe.

Changing the switch zoom to 0 is not acceptable because it would render only one regional shard across the globe.

Required repair

1. Build and publish a dedicated occumed-global-overview.pmtiles archive for zooms 0–5.
2. Use schema-compatible low-zoom layers for land, water, country boundaries, major roads, and place labels.
3. Add the overview archive URL and bounds to the worldwide manifest.
4. Update world-pmtiles-router.js so:
   • z0–5 uses the global overview archive;
   • z6+ uses the smallest matching regional archive;
   • the external OpenFreeMap source is emergency fallback only.
5. Lock the requested low-zoom visual treatment:
   • saturated green land and blue water;
   • luminous white-blue atmosphere;
   • no beige land or pink-road fallback appearance;
   • reduced label density and no limb clipping/overlap.
6. Validate the deployed globe view and regional transition before merge.

Acceptance criteria

• Whole-globe view is rendered from Occu-Med-owned PMTiles, not OpenFreeMap.
• All continents remain visible at z0–5.
• z6+ switches to the existing 754 regional PMTiles without gaps.
• Production globe matches the intended green/blue luminous styling.
• Edge labels do not stack or clip at the globe limb.