refactor(port): migrate to units::temperature based body temperature

[scarf005]

Purpose of change (The Why)

• port Body heat temperature refactor to standard units. CleverRaven/Cataclysm-DDA#68796 to replace the internal arbitrary body-temperature integer scale with units::temperature / units::temperature_delta
• legacy save/JSON/Lua BTU values should be still compatible

related: #2570, #397, #1417

Describe the solution (The How)

• body-temperature delta helpers under src/units/
• converts body-part current/converged temperature APIs and body-temperature constants to typed units
• converts legacy JSON/save/Lua BTU values at function boundaries
• deprecated *_btu Lua bindings and added Celsius bindings
• preserves BN's legacy body-temperature math at calculation boundaries while using typed int millidegree Celsius units internally

Testing

playtested for a bit, bodytemps seems to work as intended, e.g getting cold or warm

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.

Optional

• This PR used AI assistance.
  • I disclosed it in the PR description and added an Assisted-by: trailer to every AI-assisted commit in the Linux kernel format.
• This PR ports someone else's contribution (e.g from DDA or other fork).
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  • I have attributed original authors in the commit messages adding Co-Authored-By in the commit message.
  • I have linked the URL of original PR(s) in the description.
• This is a C++ PR that modifies JSON loading or behavior.
  • I have documented the changes in the appropriate location in the docs/ folder.
  • If documentation for this feature does not exist, please write it or at least note its lack in PR description.
  • New localizable fields need to be added to the lang/bn_extract_json_strings.sh script if it does not support them yet.
  • If applicable, add checks on game load that would validate the loaded data.
  • If it modifies format of save files, please add migration from the old format.
• This PR modifies lua scripts or the lua API.
  • I have added lua scope to the PR title.
  • I have added type annotations to functions so that it's safe and easy to maintain.

_{PR opened by gpt-5.5 high on pi}

Build Artifacts

PR build for commit 1f2468b (Merge branch 'main' into refactor/body-temperature-units) on 2026-07-24 01:52:41

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