NaN Handling

[jharwood0]

Description

This pr identifies use cases where NaNs result in unexpected behavior:

• Comparator treats NaN != NaN (IEEE) so two files with identical bits compare as different
• NaNs round-trip fine through Double/Real columns, but Int/Bit cols reinterpret the NaN's 8 bytes through the integer codec (with narrowing or offset arithmetic depending on which one), and decode back as a plausible-looking integer — ultimately casting {1.0, NaN, 3.0} to an INT column reads back {1, 1, 3}
• min/max column metadata also gets corrupted:
  • For Double/Real types — when a NaN arrives before any real values: NaN poisons the min/max slots, subsequent real values can't update them (NaN comparisons are always false)
  • For Integer/Bitfield — NaN bits get reinterpreted as a very large int, always overwriting max regardless of position; knock-on effect for the optimiser which sees a range of ~4.6e18

to fix the above:

• Comparator::same() now checks for NaN first — two NaNs are treated equal,
  one NaN still compares false. dropped the $odc_NAN_IS_OK env var that was
  guarding the flag-controlled version

• gatherStats early-returns on NaN, so min/max only track real values —
  fixes the Double/Real poisoning and stops Int/Bit from reinterpret-casting
  the NaN bits as a huge int

• NaN writes to Int/Bit columns are coerced to the column's existing
  missing-value sentinel with a log warning

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

📖 Documentation 📖
https://sites.ecmwf.int/docs/dev-section/odc/pull-requests/PR-52

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.31507% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (424ce3d) to head (6015bc5).

Files with missing lines	Patch %	Lines
src/odc/Comparator.h	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #52      +/-   ##
===========================================
+ Coverage    86.91%   87.02%   +0.11%     
===========================================
  Files          188      189       +1     
  Lines        15022    15157     +135     
  Branches      1363     1373      +10     
===========================================
+ Hits         13056    13191     +135     
  Misses        1966     1966              

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