Bound tiled raster viewport admission

[EthanOConnor]

What

• admit each tiled-raster key at most once per stable viewport
• derive a bounded admission budget from decoded tile size and cache capacity
• choose only origin-aligned overview levels, coarsening oversized views when possible
• stop requests entirely when an odd-origin source has no correctly registered overview at the current scale
• add regressions for cache churn, odd TMS origins, and adaptive overview selection

Why

A large visible window could exceed the raster cache, causing freshly evicted tiles to be requeued indefinitely. Cropped TMS sources with odd tile origins also forced overview selection back to full resolution, amplifying the churn into thousands of tiny network requests and an apparent application wedge.

Impact

Stable viewports now converge after bounded work. Large aligned sources use a fitting overview; sources that cannot render a correctly aligned overview wait until the user zooms in instead of fetching a partial carpet of full-resolution tiles.

Checks

• full macOS suite: 36/36 passed
• focused gdal_tiled_t: passed, including three new regressions
• live Kelsey Creek/Wilburton aligned source: imagery loaded and process settled idle
• live disposable odd-origin source: no tile network requests at the oversized scale and process settled idle