feat: narrow partial reads of uncompressed chunks within a single row

[bendichter]

Summary

Stacked on bytes-partial-decode (zarr-developers#4458). This PR is against that branch in the fork so the diff shows only the new change.

The parent PR reads the rows along the first axis that a selection touches. This extends it to the case where the selection touches a single row: the same row window is then applied along the next axis, and so on. For a (T, Y, X) chunk, arr[t, y0:y1, x0:x1] fetches rows y0 to y1 of frame t instead of the whole frame, and arr[t, y, x] fetches one item.

For example, say you have a stack of image frames through time (i.e. a video): (T, Y, X), and you wanted a region of a single frame.

• Previously: The entire chunk is read, potentially reading many frames you don't need.
• bytes-partial-decode: Only the desired row (frame) is read, but you must read the entire row (frame)
• This PR: This will read a region of the desired frame, sub-selecting only the rows of interest (but only if a single frame is selected)

How it works:

• _axis_window is the per-axis logic that _row_window had: the first and last index a selector touches, and the selector shifted to start at zero.
• _block_window walks the axes from the first. It records each axis's window and continues to the next axis only while the window is a single index. It returns the block's offset in items, the block's shape, and the shifted selection.
• The block keeps the narrowed axes with length 1, so it has the same number of axes as the chunk and the shifted selection applies to it unchanged.
• A selection that touches more than one row is read exactly as before. Trimming the first and last row of a multi-row window would save less than one row and the fetched bytes would no longer be a rectangular block.

For reviewers

• The exact-length handling of store responses from the parent PR is unchanged; it now works on the block's byte range.
• test_block_window checks, for each case, that the shifted selection picks the same items from the block as the original selection does from the chunk.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/ (extends the parent PR's changes/4458.feature.md; needs its own file if opened upstream separately)
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

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