Don't scale binary table columns without TSCAL/TZERO

[barrettp]

Problem

When a binary table column has no TSCAL/TZERO, the defaults are 1.0f0/0.0f0 (Float32), and the readers compute field.zero .+ field.scale.*data. That promotes integer data to Float32, so Int32 values above 2^24 are silently rounded:

stored	read
16777217	16777216
16777275	16777276
33554433	33554432
2000000001	2000000000

For a variable-length (PJ) column the Float32 result is converted back into the Vector{Int32}, so the type looks right but the values are wrong. Fixed-width integer columns come back as Float32. This corrupted the pointer table of XSTAR's atdb.fits (~1M of 1.2M offsets), which astropy reads exactly.

Fix

At the top of both read(::IO, ::BinaryField, ...) methods, set scale = scale && !(field.scale == 1 && field.zero == 0) so columns with no scaling pass through unchanged. Columns with an actual TSCAL/TZERO behave as before.

This changes the output type of unscaled integer columns: they are returned as stored (Int32 stays Int32) instead of Float32.

Tests

• New regression test test/data/int32_large.fits (written with astropy; fixed 8J and variable-length PJ columns around 2^24). It fails without the change and passes with it.
• The existing suite passes.
• Checked on the real atdb.fits: all 1,216,791 real-array offsets match a running sum of the record counts, as astropy reads them.

The image, primary, ASCII table and random-groups readers use the same zero .+ scale.*data pattern and are not changed here.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.53%. Comparing base (870e4ee) to head (18709d2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   46.35%   47.53%   +1.17%     
==========================================
  Files          17       17              
  Lines        2550     2552       +2     
==========================================
+ Hits         1182     1213      +31     
+ Misses       1368     1339      -29     

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