fix: destroy tile textures on eviction (#591)

[kylebarron]

We need to manually destroy any GPU textures that we create ourselves. This applies both to library code and to app examples

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Summary

Frees per-tile GPU textures on cache eviction, closing a GPU-memory leak that accumulated while panning/zooming. A luma.gl Texture holds a WebGL handle that JS GC does not deterministically reclaim, and deck.gl's onTileUnload docs state the caller owns anything returned from getTileData — passing a texture to a sublayer does not transfer ownership.

The fix follows a single ownership principle: whoever creates a texture frees it.

• Library default pipelines now clean up their own textures. A new protected _onTileUnloadCallback() hook on RasterTileLayer (mirroring _getTileDataCallback() / _renderTileCallback()):
  • COGLayer destroys tile.data.texture + mask, but only when its default pipeline ran (no user getTileData).
  • MultiCOGLayer always destroys every band texture.
  • ZarrLayer inherits the no-op base — it requires a user getTileData, so its textures are user-owned.
• Examples with a custom getTileData own their textures, so each frees them in its own onTileUnload one-liner: usgs-topo-cutline, vermont-cog-comparison, land-cover, naip-mosaic, dynamical-zarr-ecmwf, nldas-icechunk, aef-mosaic. Shared colormap/filter textures are module-level and intentionally left alone.
• zarr-sentinel2-tci was audited: its tile data is CPU ImageData and the GPU texture is created and owned by deck.gl's type: "image" prop handling, so no cleanup is needed (documented with a comment).

An internal destroyIfTexture helper guards with instanceof Texture; library cleanup composes with (runs after) any user-supplied onTileUnload. No new public API.

Design spec: dev-docs/specs/2026-06-05-destroy-tile-textures-design.md.

Closes #591. Closes #135

Test Plan

• destroyIfTexture unit tests (destroys Textures, ignores absent/non-Texture)
• COGLayer composes cleanup only when no getTileData; passthrough otherwise
• MultiCOGLayer always composes band cleanup
• Base RasterTileLayer returns user onTileUnload unchanged
• Full suites: deck.gl-raster (137) + deck.gl-geotiff (31) green
• Workspace pnpm typecheck and pnpm biome check clean

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Written by Claude on behalf of @kylebarron

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