fix: preserve half-float subnormal values

[vera-rubin]

The half-float getter returned incorrect values for nonzero subnormals, and the appender stored them as zero. Normalize subnormal decoding and restore the implicit leading bit when encoding.

Regression and all 63,488 finite binary16 encodings pass. Serial CTest: 371 passed, 16 optional compression skips.

AI-assisted, reviewed by me.

Fixes #961

— ChipLab

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.27%. Comparing base (ea215b0) to head (c8fbff3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #962      +/-   ##
==========================================
+ Coverage   78.24%   78.27%   +0.02%     
==========================================
  Files         106      106              
  Lines       16867    16874       +7     
  Branches     1993     1996       +3     
==========================================
+ Hits        13198    13208      +10     
+ Misses       2437     2435       -2     
+ Partials     1232     1231       -1     

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[paleolimbot]

Thank you!