[scripts] Treat missing legacy raster CRS as a warning

[Chain-Frost]

Problem

Legacy TUFLOW FLT and ASC outputs may omit .prj files. PR #107's strict CRS check incorrectly reports those as mismatches even when cell values, NoData masks, and grid alignment are identical.

Change

• Compare declared CRS definitions only when both files have one. A disagreement between two declared CRSs still fails validation.
• When either/both rasters omit CRS metadata, continue checking all raster values, masks, dimensions and grid transform; after a successful comparison mark the result as MATCH_CRS_UNVERIFIED (eligible for the deletion proposal, with explicit uncertainty).
• Split actual errors from missing-CRS warnings. Add non-overwriting tuflow_raster_conversion_warnings.csv; include status and warning columns on proposed deletions.
• No change to TIF-newer requirement, explicit delete confirmation, metadata safeguards, symlink checks, or retention of .prj files.
• Document the distinction in the TUFLOW scripts README.

Tests

• New tests for FLT/ASC with missing source CRS but declared TIF CRS, both rasters missing CRS, TIF CRS missing, declared CRS disagreement, genuine pixel mismatch when CRS is missing, and confirmed deletion of a warning candidate.
• Existing synthetic raster test suite retained.

Caution

A warning indicates that the spatial reference is not independently proven, even when stored grid geometry and pixel values match. Review any warning-labelled deletion candidates before approving deletion. No project data was accessed or changed.