Preserve CLI unit spellings in Clap support

[JordanNoone]

Enabling the unit enums' new clap feature rejects existing abbreviations such as m, kg, and kg:m3, including the CLI's own default arguments. UnitVolume also lacks ValueEnum, and enabling this feature alone fails to compile because Clap's required std feature is disabled.

Preserve the existing 31 unit spellings and density aliases, add ValueEnum to UnitVolume, and enable clap/std. Parsing stays on the modeling-command enums without API-to-modeling conversions. Clap remains optional.

This PR completes the Clap unit support merged in #1369, targets main, and supports KittyCAD/cli#1844.

Validation:

• Two focused parsing tests pass, covering all six unit enums and density aliases.
• cargo +1.98.1 check -p kittycad-modeling-cmds --no-default-features --locked --offline passes.
• cargo +1.98.1 clippy -p kittycad-modeling-cmds --features clap --all-targets --locked --offline -- -D warnings passes.
• Formatting passes.
• The CLI consumer's seven focused tests pass with this fix; three of those tests fail against the unmodified UoM can be parsed by Clap #1369 head because volume diagnostics and analysis defaults are broken.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             main    #1370   +/-   ##
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  Hits            ?      664           
  Misses          ?     1194           
  Partials        ?        0           

Flag	Coverage Δ
unittests	35.73% <100.00%> (?)

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[JordanNoone]

@adamchalmers ready for review. Resolved the conflict with main after #1369 landed.

Changes here:

• Added Clap ValueEnum support to UnitVolume.
• Preserved existing unit spellings (mm3, m2, kg, etc.) and density aliases so current arguments and defaults keep working.
• Enabled Clap’s required std feature while keeping the dependency optional.
• Added regression coverage for all six unit enums and density aliases.

KittyCAD/cli#1844 now parses directly into the modeling-command enums and removes the conversion helper, as requested. All 11 checks pass on efad232.