Reimplement the FITS parser natively, without kerchunk

[TomNicholas]

Parses FITS with astropy directly and builds the ManifestStore from the HDU offsets it reports, instead of routing through kerchunk's reference dict and translating it back. The fits extra now installs only astropy.

Follows the same pattern as the native netCDF3 parser: read the format's own structure, build ManifestArrays from it, skip the round-trip through kerchunk refs entirely.

Coverage went up

ASCII tables (TABLE HDUs) previously could not be read at all — get_codec_class("numcodecs.FITSAscii") raised KeyError, so they failed at metadata construction. This adds a real zarr v3 FITSAsciiTableCodec. The Hubble file used by the existing test turned out to contain an ASCII table that kerchunk was silently skipping; all six of its columns now match astropy exactly.

Three correctness bugs found along the way:

• BSCALE was inverted. It was passed straight to FixedScaleOffset, which divides on decode, so every scaled image came back scaled by 1/BSCALE.
• ASCII column layout was wrong. The previous approach assumed columns tile the row with a one-byte separator. They don't — TBCOL places them, and the real Hubble table has gaps (offsets [0, 16, 32, 48, 64, 92], itemsize 104). Parsing also has to allow the Fortran D exponents the format uses, which numpy cannot parse; without that the real file fails outright.
• BLANK was ignored, despite custom_parsers.md documenting that FITS parsers should map it. It now becomes a _FillValue, scaled alongside the data when BSCALE/BZERO are present.

Behaviour changes

• Every data HDU is exposed, not just the first.
• Axes are named per-HDU (SCI_y, SCI_x) rather than a shared y/x. The shared names only worked because kerchunk emitted a single extension; with several HDUs they collide, since HDUs routinely disagree about the length of a given axis.
• reader_options is gone. The parser reads through the ObjectStoreRegistry rather than fsspec, so fsspec storage_options no longer apply — configure credentials on the store you register.

Binary tables now raise

Their columns form a big-endian structured dtype, and the zarr v3 struct data type has no way to record a field's byte order: [('flux', '>f4')] serializes to plain float32 and reparses little-endian, and zarr's bytes codec does not apply endian to a structured dtype anyway. So any virtual reference to big-endian record data reads back byte-swapped once the metadata has been through zarr.json.

This is pre-existing and not specific to FITS — it reproduces with a hand-built ManifestArray and no FITS involved. The parser previously accepted these HDUs and returned wrong numbers; it now refuses them, and skip_variables excludes them. Worth fixing properly at the zarr level, but that is a separate change with a much wider blast radius.

Testing

Local fixtures cover images, cubes, scaled images, ASCII tables, BLANK masking, per-HDU axis naming, duplicate EXTNAME disambiguation, and the binary-table refusal, so the parser is no longer only exercised by a network test. The two network tests validate against the real Hubble file, including the previously-skipped ASCII table.

The existing Hubble test passed region="us-west-2", but the bucket is us-east-1. It went unnoticed because kerchunk read via fsspec and the registered store was never actually used; now that the parser reads through it, the region has to be right.

[codecov]

Codecov Report

❌ Patch coverage is 96.96970% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (7805559) to head (ce3d802).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
virtualizarr/codecs.py	95.34%	2 Missing ⚠️
virtualizarr/parsers/fits.py	97.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1090      +/-   ##
==========================================
+ Coverage   89.67%   90.04%   +0.37%     
==========================================
  Files          41       41              
  Lines        2682     2803     +121     
==========================================
+ Hits         2405     2524     +119     
- Misses        277      279       +2     

Files with missing lines	Coverage Δ
virtualizarr/codecs.py	94.78% <95.34%> (+0.26%)	⬆️
virtualizarr/parsers/fits.py	97.89% <97.75%> (-2.11%)	⬇️

... and 1 file with indirect coverage changes

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