feat: Support variable chunking

[maxrjones]

Note: Mostly a proof of concept now to facilitate upstream reviews, but still opening for visibility

Overview

• Add initial support for RectilinearChunkGrid (variable-length chunks), enabling VirtualiZarr to work with archival formats that have non-uniform chunk sizes (e.g., stripped TIFFs where image height isn't evenly divisible by rows per strip)
• Remove the RegularChunkGrid-only restriction from ManifestArray and update the concatenation/stacking machinery to handle rectilinear chunk grids
• Add a test that creates two stripped TIFFs with rectilinear chunks via virtual-tiff and concatenates them with xr.concat

Details

Core changes:

• ManifestArray.chunks now returns chunk_grid.chunk_shapes (tuple of tuples) for RectilinearChunkGrid, rather than raising NotImplementedError. Uses the same compatibility pattern as xarray: check not isinstance(chunk_grid, RegularChunkGrid) behind a has_rectilinear_chunk_grid_support feature flag, so it falls back gracefully on older zarr versions
• copy_and_replace_metadata detects nested chunk sequences (via vendored _is_nested_sequence) and creates the appropriate chunk grid type (regular vs rectilinear)
• stack correctly inserts a single-element tuple (1,) when adding a new axis to a rectilinear array
• check_no_partial_chunks_on_concat_axis skips the check for rectilinear grids, which explicitly encode variable chunk sizes
• Vendored _is_nested_sequence from zarr-python#3534 into virtualizarr/vendor/zarr/core/chunk_grids.py for detecting rectilinear chunk representations

Compatibility:

• All changes are backward-compatible with zarr 3.1.x (no RectilinearChunkGrid)
• The test is skipped when zarr lacks RectilinearChunkGrid support
• mypy passes cleanly on the standard test environment

Environment:

• Added rectilinear pixi environment with forked zarr-python, xarray, and virtual-tiff branches

Checklist

• Closes Support variable-length chunks #12
• Depends on Rectilinear (variable-length) chunk grid zarr-extensions#25, variable-sized chunks with zarr v3 pydata/xarray#10880, Feature: support rectilinear chunk grid extension  zarr-python#3534
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[codecov]

Codecov Report

❌ Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.93%. Comparing base (ba1f464) to head (bb7dc7e).

Files with missing lines	Patch %	Lines
virtualizarr/vendor/zarr/core/chunk_grids.py	71.42%	4 Missing ⚠️
virtualizarr/manifests/utils.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #877      +/-   ##
==========================================
- Coverage   89.11%   88.93%   -0.18%     
==========================================
  Files          34       35       +1     
  Lines        1956     1980      +24     
==========================================
+ Hits         1743     1761      +18     
- Misses        213      219       +6     

Files with missing lines	Coverage Δ
virtualizarr/manifests/array.py	86.02% <100.00%> (+1.07%)	⬆️
virtualizarr/manifests/array_api.py	96.96% <100.00%> (+0.09%)	⬆️
virtualizarr/manifests/utils.py	90.47% <83.33%> (-0.27%)	⬇️
virtualizarr/vendor/zarr/core/chunk_grids.py	71.42% <71.42%> (ø)

... and 1 file with indirect coverage changes

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