Pool argument states to reduce application size

[eiriktsarpalis]

Summary

Reduce Native AOT generic specialization by using pooled class-backed argument states for source-generated and Reflection.Emit constructors, methods, and functions.

• Add IArgumentState.Return() and PT0033 to enforce constructor-state cleanup; generated function wrappers return states in finally.
• Keep plain reflection and the F# fallback non-pooled through a private ReflectionArgumentState.
• Retain the previous struct helpers as obsolete compatibility types.
• Expand the .NET 11 size canary to 75 record types and 135 generated shapes, exercising JSON and configuration binding. Refresh all three platform baselines from native CI measurements.

Native AOT size improvement

The representative canary is 24.55–25.57% smaller than main (45dd35c). Each platform builds both library versions with identical application source and publish settings:

Platform	Main libraries	This PR (9a0be57)	Reduction
Linux x64	6,590,120 B	4,972,504 B	24.55%
Windows x64	5,852,672 B	4,380,160 B	25.16%
macOS ARM64	6,435,328 B	4,789,840 B	25.57%

Native measurements use the pinned .NET 11 RC SDK, Release size optimization, invariant globalization, and disabled stack traces. Each runner repeated publishes in reverse order with identical sizes and successful JSON/configuration round trips. This is a file-size comparison, not a throughput claim.

Validation

make test, make pack, and make test-aot-size pass. CI is green across all six Linux/Windows/macOS Debug/Release jobs, including native-size checks. Markdown and Codecov checks also pass.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.33%. Comparing base (45dd35c) to head (9a0be57).

Files with missing lines	Patch %	Lines
...Generator/Analyzers/ArgumentStateReturnAnalyzer.cs	87.75%	7 Missing and 5 partials ⚠️
...ConfigurationBinder/ConfigurationBinder.Builder.cs	82.35%	3 Missing ⚠️
...SourceGenerator/SourceFormatter/SourceFormatter.cs	75.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #521      +/-   ##
==========================================
+ Coverage   94.29%   94.33%   +0.03%     
==========================================
  Files         255      258       +3     
  Lines       17277    17516     +239     
  Branches     3361     3393      +32     
==========================================
+ Hits        16292    16523     +231     
- Misses        747      751       +4     
- Partials      238      242       +4     

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