Add a spec-only nanmedian fallback for Combiner.median_combine

[mwcraig]

Adds ccdproc/_nanmedian.py, a NaN-aware median written only in terms of array-API-standard functions (sort the axis, count non-NaN entries, gather and average the two middle elements; all-NaN slices give NaN; integer input is promoted to float64). Combiner._default_median now returns this fallback (bound to the namespace via functools.partial) when the namespace has no nanmedian, instead of raising RuntimeError. The fallback is O(n log n) along the combination axis; native nanmedian / bottleneck are still preferred when available.

Also:

• docs/array_api.rst: replace the claim of a bottleneck -> numpy fallback (which did not exist) with the real chain (bottleneck for numpy -> xp.nanmedian -> spec-only fallback).
• test_combiner.py: drop the numpy-median/.compute() workarounds and the pytest.skip for namespaces without median; test_bottleneck_defaults_respect_array_namespace now checks the fallback is returned when nanmedian is absent.
• New ccdproc/tests/test_nanmedian.py (odd/even lengths, 1-D/2-D/3-D, partial and all-NaN columns, integer input, axis=1/-1, unsupported axis).

Test matrix (pytest ccdproc):

• numpy: 393 passed, 5 skipped
• jax: 379 passed, 10 skipped, 2 xfailed, 7 xpassed (the 7 XPASS are identical on upstream/main; astropy/jax DeprecationWarning markers, unrelated)
• dask (with CCDPROC_LOG_ARRAY_ESCAPES=1 CCDPROC_ENFORCE_ESCAPE_BASELINE=1): 383 passed, 15 skipped; no escapes outside the baseline
• array-api-strict, test_nanmedian.py: 13 passed
• array-api-strict, test_combiner.py: 75 failed / 12 passed before -> 74 failed / 13 passed after. All remaining median_combine failures under strict hit the Combiner constructor ("Nested Arrays are not allowed", being fixed in Build Combiner data and mask arrays with xp.stack #976) or other known strict bugs, not the median itself.

Fixes #906

🤖 Generated with Claude Code

https://claude.ai/code/session_01DTN9DnPnLKK2u7knnMJ2gA

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.84%. Comparing base (89e90f9) to head (7c96d93).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
ccdproc/combiner.py	50.00%	2 Missing ⚠️
ccdproc/_nanmedian.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
+ Coverage   95.83%   95.84%   +0.01%     
==========================================
  Files           8        9       +1     
  Lines        1631     1659      +28     
==========================================
+ Hits         1563     1590      +27     
- Misses         68       69       +1     

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