Generate GetDefaultMappings as an array instead of an iterator

[tmilnthorp]

Each generated GetDefaultMappings() used yield return. The compiler turns each one into a hidden iterator class, so that's 129 extra classes in the DLL. This PR generates an array instead. The return type stays IEnumerable<UnitDefinition<TUnit>>, so there's no API change. The generated code reads the same apart from the array syntax.

QuantityInfo calls .Select(...).ToArray() on the mappings. Given an array, LINQ knows the count up front and allocates the result once.

The generator change and the regenerated code are separate commits.

	master	this PR
UnitsNet.dll (net10.0, Release)	1,885,696 B	1,691,648 B (−10.3%)
First ToString() (cold process)	355 ms	332 ms
Create all 129 QuantityInfos (cold process)	339 ms / 1,565 KB allocated	318 ms / 1,458 KB allocated

Hot-path benchmarks for ToString, Parse, conversions, arithmetic and QuantityInfoLookup lookups are unchanged within noise.

🤖 Generated with Claude Code

https://claude.ai/code/session_012XKhDsyHDc5BHrmibxScqG

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (3bf1ef4) to head (6927559).

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1737     +/-   ##
========================================
- Coverage      98%     98%     -1%     
========================================
  Files         515     515             
  Lines       24092   22361   -1731     
========================================
- Hits        23692   21961   -1731     
  Misses        400     400             

Flag	Coverage Δ
net48	97% <100%> (-1%)	⬇️

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