[LJimmy007]

Related to #348; this advances only the BitRound subtask.

BitRound's current tests exercise data produced by the current encoder. Add small
fixed compatibility data for float32/float64 and two explicit keepbits values,
then decode it using the instance returned by get_codec after a JSON roundtrip.
Assert the restored type/configuration separately from dtype, shape and exact
rounded values, since decoding does not depend on keepbits.

The four binary fixtures total 96 bytes and were established from commit
1f83681cf310ac9aac8041ed6ff4a5dcc42a7aad. Their README records the input,
configuration, byte order, generation procedure and independently calculated
expectations. Tests only read these files and fail when required data is absent.
This adds compatibility coverage without changing production behavior, the
shared compatibility helper, or requirements on future encoder output.

Validation on Windows x86_64, Python 3.13.5, NumPy 2.5.4, with this checkout and
its C/Cython extensions built through Meson:

• Focused BitRound/registry/compatibility tests: 34 passed.
• Full suite including Zarr 3.1.6: 874 passed, 21 skipped, 1 expected failure.
  pytest -p no:zarr produces the same result while avoiding Zarr's pytest plugin
  importing Numcodecs before coverage starts. BitRound and registry coverage is
  100%; total Python coverage is 99.34% (alternate CRC32C backend branches remain).
• All applicable configured pre-commit hooks pass.
• HTML documentation builds with three warnings also present in an unmodified
  baseline copy; no new documentation warnings.
• Local fault probes confirm missing config/data, wrong restored keepbits, and
  wrong decoded dtype/shape/values fail at the intended checks. After removing
  substitutions, all four cases pass with encoding forbidden and fixture hashes
  unchanged. These probes are outside the proposed patch.

The upstream cross-platform CI matrix has not been run for this patch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1f83681) to head (8dd3a6e).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #875   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          905       905           
=========================================
  Hits           905       905           

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[LJimmy007]

A quick update: the upstream Tests, Wheels, and i386 workflows have now passed for this PR. This is limited to BitRound compatibility fixtures and registry-restored decoding tests, with no production behavior changes.

Could a maintainer take a look when convenient? I'd appreciate feedback on whether this coverage is a suitable step toward #348. Thank you!