Refactor GeoTIFF Phase 2: centralize transform/georef contract (PR-B of #2211)

[brendancol]

Summary

Part of epic #2211. Phase 2 deliverable: one shared transform/georef resolver that every backend (read and write) calls, with parity tests across the supported cases. Replaces hand-copied coord-to-transform and georef classification logic that currently lives in multiple modules.

Scope

Refactor + parity tests. No public API change.

Files

• Modify: xrspatial/geotiff/_coords.py — promote/centralize coords_to_transform, add resolve_georef() that returns a typed result.
• Modify: xrspatial/geotiff/_attrs.py — georef_status derivation routes through resolve_georef().
• Modify: xrspatial/geotiff/_writers/eager.py, _writers/gpu.py, _writers/vrt.py — call the resolver instead of inline logic.
• Modify: xrspatial/geotiff/_backends/vrt.py — call the resolver.
• Create: xrspatial/geotiff/tests/test_georef_resolver_parity_2211.py.

Resolver contract

• Input: a DataArray, or coord arrays + crs hint + transform hint.
• Output: a GeorefResolution dataclass:
  • transform: GeoTransform | None
  • georef_status: Literal["none","coords","transform-only","crs-only","rotated-dropped","full"]
  • dropped_reason: str | None
  • applied_no_georef_marker: bool
• Owns: coord-to-transform inference, no-georef marker behavior, degenerate axis policy, rotated-affine read-only/drop policy.

Parity tests

Cover at minimum:

• y/x dim names
• lat/lon dim names
• row/col dim names (no georef)
• transform-only (no CRS)
• CRS-only (no transform)
• rotated-affine dropped on read

For each case, assert identical GeorefResolution produced from every supported call site.

Acceptance

• All 6 fixture cases produce identical GeorefResolution across eager/Dask/VRT call sites.
• xrspatial/geotiff/tests/test_georef_status_2136.py and the test_allow_rotated_* suites still pass.
• No call site outside _coords.py / _attrs.py builds a GeoTransform by hand on the read or write paths covered.
• No public API change.

Closes part of #2211.