feat: Support rectilinear chunk grids - #69

Summary

• Support TIFFs where image height is not evenly divisible by rows-per-strip, using zarr's rectilinear chunk grid instead of erroring out
• Add a TruncateCodec to handle edge strips that are padded to full size on disk
• Remove ~30 previously-xfailed test files

Details

Previously, strip-based TIFFs with partial final strips (e.g., 100px height with 30px rows-per-strip) raised a ValueError because zarr's regular chunk grid requires uniform chunk sizes. This PR uses rectilinear chunk grids instead, allowing the final strip to have a different size.

Key changes:

• parser.py: _get_chunks_from_strips now returns a nested sequence (e.g., [[30, 30, 30, 10], [width]]) for non-uniform strips instead of raising. _construct_manifest_array selects between regular and rectilinear chunk grids accordingly.
• codecs.py: New TruncateCodec (bytes-to-bytes) trims oversized edge-chunk buffers to their expected logical size before downstream codecs process them.
• utils.py: Added _is_nested_sequence helper (vendored from zarr-python#3534) to distinguish rectilinear chunk specs from regular ones.
• test_gdal_examples.py: Removed the entire partial_chunks xfail list (~30 files). Moved a few files back into their correct category lists (jpeg_tables, YCbCr).

Dependencies

Requires upstream rectilinear chunk grid support in zarr-python, VirtualiZarr, and xarray (see pyproject.toml for feature branches).