Epic: Conservative VRT support contract for GeoTIFF release

[brendancol]

Goal

Keep VRT support for real user workflows, but make it a conservative advanced feature with a documented, enforced subset.

VRT is valuable for mosaics and interoperability, but full GDAL VRT parity is not a realistic release promise for this module. The release should support simple mosaics well and fail closed outside that subset.

Supported For This Release

• Simple GDAL VRT mosaics backed by GeoTIFF sources.
• Compatible source CRS, dtype, transform orientation, pixel size, band count, and band layout.
• Windowed reads where source and destination windows map cleanly.
• Lazy/dask reads over the same supported subset.
• Explicit nodata handling.
• Mixed-band nodata rejection by default, with only documented opt-ins.
• missing_sources='raise' as the default behavior.

Not Promised For This Release

• Full GDAL VRT compatibility.
• Warped VRTs / reprojection VRTs.
• Nested VRTs.
• Arbitrary resampling semantics beyond the implemented and tested subset.
• Mixed CRS, mixed dtype, mixed resolution, or mixed band metadata unless explicitly supported.
• Complex mask/alpha/source semantics not represented in the GeoTIFF attrs contract.

Work Items

• Add a VRT capability validator that runs after VRT parse and before read execution where feasible.
• Centralize validation so read_vrt and open_geotiff(... .vrt ...) fail consistently.
• Validate CRS compatibility, dtype compatibility, source/destination rect sanity, band count, nodata policy, transform compatibility, and resampling support.
• Add tests for supported simple mosaics.
• Add negative tests for unsupported VRT features.
• Ensure missing-source behavior is explicit and tested for raise and warn.
• Add docs and docstrings that describe VRT as advanced simple-mosaic support, not GDAL replacement.

Acceptance Criteria

• Supported simple VRT mosaics work through eager and dask paths.
• Unsupported VRT features fail with actionable errors.
• Mixed or ambiguous metadata cannot silently flatten into one output raster.
• VRT tests assert values, coords, and attrs.
• Documentation plainly states the VRT subset and non-goals.