Geotiff polish: validation, caching caps, parallelism thresholds, memory guards (#1488)

[brendancol]

Fixes #1488.

What's in here

Ten small ergonomic / safety items from the geotiff audit, bundled.

• C-1: to_geotiff validates compression up-front instead of letting the error surface from _compression_tag. Same exception class, friendlier traceback.
• C-2: Documented the read_geotiff_dask -> read_vrt defensive fallback. No public behavior change.
• C-5: write_vrt docstring spells out relative, crs_wkt, nodata instead of pointing at the internal helper.
• C-6: Predictor doc lists True/2 (horizontal), False/0/1 (none), 3 (fp, float dtypes).
• C-7: tiled=False with a non-default tile_size now warns instead of silently ignoring.
• P-3: _mmap_cache got a soft LRU cap (default 32, configurable via XRSPATIAL_GEOTIFF_MMAP_CACHE_SIZE). In-use entries are never evicted; only idle ones drop when the cap is exceeded.
• P-4: Decode parallelism now triggers on n_tiles > 1 and tile_pixels > 64*1024 regardless of compression. Uncompressed multi-tile reads previously skipped the pool.
• P-5: read_geotiff_dask now raises ValueError (with a hint) when chunks would produce more than 50,000 dask tasks, instead of silently rescaling at 1M. The hint suggests a chunk size that fits.
• P-6: _gpu_decode._check_gpu_memory(required_bytes) calls cupy.cuda.runtime.memGetInfo() and raises MemoryError when the request exceeds 90% of free VRAM. Wired into the four largest allocations (nvJPEG output buffer, full-image output buffer, tile decode buffer x2).
• P-9: COG auto-overview generation capped at 8 levels (1/256 resolution). Explicit overview_levels=[...] is not capped.

Behavior changes

• to_geotiff(..., compression=<bad>) now raises with "Unknown compression" rather than "Unsupported compression". Existing test updated to match either phrasing.
• to_geotiff(..., tiled=False, tile_size=<non-default>) now emits a UserWarning.
• read_geotiff_dask raises instead of auto-scaling past 50K tasks. Callers relying on the old silent rescale will need to pick chunks explicitly.

Test plan

• 24 new tests in test_polish_1488.py, one cluster per item
• All 493 pre-existing geotiff tests still pass (4 unrelated matplotlib failures from the env, not from this change)
• Touched test (test_unsupported_compression) updated to accept either error phrasing