Add free-threaded tests with pytest-run-parallel

[kylebarron]

Follow up from #69

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Claude:

Follow-up: free-threaded (3.14t) CI testing

PR #69 adds the wheel builds for free-threaded CPython 3.14 (cp314-cp314t)
and scaffolds the abi3t (3.15+) wheel. It intentionally does not add CI that
tests zarrista on a free-threaded interpreter — this issue tracks that work,
which turned out to need a test-suite restructure rather than a CI one-liner.

Why it was deferred

Spiking the 3.14t test job surfaced three blockers:

1. numcodecs re-enables the GIL. numcodecs.blosc hasn't declared that it
   can run without the GIL, so importing it on 3.14t silently turns the GIL back
   on:

   RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'numcodecs.blosc' ...

   Since nearly all tests pull in zarr-python (→ numcodecs), running the current
   suite on 3.14t gives a false free-threaded signal — the GIL is on.

2. Missing free-threaded wheels → slow / degraded. icechunk and
   numcodecs compile from source on 3.14t (no cp314t wheels yet), and
   google-crc32c (via icechunk) falls back to its pure-Python implementation:

   As the c extension couldn't be imported, google-crc32c is using a pure python implementation that is significantly slower.

3. Tests use zarr-python as the fixture writer. 8 of 10 test files call
   zarr.create_array / zarr.open_group to produce fixtures, then read them
   back with zarrista. These are really interop/integration tests. We have
   almost no coverage that exercises zarrista's own read+write round-trip
   (ArrayBuilder.create / store_chunk / store_metadata → Array read).

Proposed plan

• Split the suite into own tests (zarrista-only read/write) vs
  integration tests (zarr-python / icechunk / arro3 interop), via markers
  or a tests/integration/ directory.
• Add zarrista-only round-trip tests (create + store chunks + read back,
  compared against numpy) so there's real own-code coverage.
• Split dependency groups so the free-threaded job installs only GIL-safe
  deps (avoid zarr/numcodecs/icechunk).
• Add a 3.14t CI job that runs only the own tests on the free-threaded
  interpreter.
• Revisit pytest-run-parallel for genuine thread-safety stress — note it
  runs the same test in N threads sharing one fixture, so any test that
  mutates shared store state flakes; concurrency tests must use per-thread
  isolated stores.
• Re-enable integration tests on 3.14t once numcodecs/icechunk ship
  free-threaded wheels and declare GIL-safety.