refactor(geo-layers): prioritize tile requests by viewport center

[charlieforward9]

geo-layer-center-priority.mov

Goals

• Improve TileLayer request scheduling so the viewport center loads first during pan, zoom, and globe transitions.
• Make center-priority scheduling the default behavior, without adding a public request-priority accessor.

Changes

• Adds internal request-priority plumbing from Tileset2D to Tile2DHeader so queued requests can be ordered by the active viewport.
• Prioritizes queued, relevant tiles by viewport-center distance; tiles containing the center get the best score, and stale queued tiles still return negative priority so they can be cancelled.
• Documents the default scheduling behavior under maxRequests.
• Adds focused tests for scheduler ordering, center-distance priority, unprojectable fallback, and stale cancellation.

Validation

• yarn test-headless test/modules/geo-layers/tileset-2d/tile-2d-header.spec.ts test/modules/geo-layers/tileset-2d/tileset-2d.spec.ts passed (22 tests).
• yarn lint passed with existing repo-wide warnings only.

[coveralls]

coverage: 83.424% (+0.03%) from 83.391% — codex/tile-center-priority into master

[chrisgervang]

Please update the PR description and address the small comments. Then should be ready

[charlieforward9]

feature -> refactor

good to go! @chrisgervang