Add transform_precision parameter to merge()

[brendancol]

Summary

• Adds transform_precision=16 to merge() and plumbs it through _merge_inmemory and _merge_dask (previously hardcoded to 16). Set to 0 for exact per-pixel transforms matching GDAL/rasterio.
• Passes the value through _validate_grid_params so the existing validation runs.
• Updates the merge() docstring with the parameter description and adds a Notes line saying the dask path should be used for very large outputs.
• max_memory is not added; merge() has no streaming path that would consume it.

Closes #1452

Test plan

• pytest xrspatial/tests/test_reproject.py -x (153 passed, including 5 new tests under TestValidateMergeGridParams)
• merge([raster], transform_precision=0) runs and produces a result
• Default transform_precision=16 (no kwarg) still works
• Negative and float values raise ValueError matching transform_precision
• transform_precision=0 and transform_precision=16 produce numerically identical merged output (rtol=1e-10) on smooth EPSG:4326 inputs

[brendancol]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in a0cc6ed. The only conflict was in _merge_inmemory — combined the per-raster r_nd nodata sentinel from main with the transform_precision parameter from this branch: resampling, r_nd, transform_precision. All 157 tests pass.

[brendancol]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in 3b37a14. The conflict was in the merge() docstring's Returns section — combined main's additions (y-coordinate order description and non-spatial coords note) with our branch's Notes section. 170 tests pass.

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