[eiriktsarpalis]

Adds PolyType.FastPathJson, a toy proof of concept showing that the PolyType.Roslyn models can drive direct JSON code generation rather than runtime type shapes.

[GenerateJson] and [GenerateJsonFor<T>] generate IJsonSerializable<T> implementations with static Utf8JsonWriter/Utf8JsonReader operations. Neither prototype project depends on PolyType.dll or the built-in generator. Shared type-naming helpers move to PolyType.Roslyn, and writer pooling is source-linked from the example serializer.

Coverage compares representative PolyType.TestCases values with the example JSON serializer and checks generator diagnostics, incrementality, composition and caller-owned readers/writers. The existing JSON benchmarks gain the prototype as a comparator, with refreshed charts. The prototype README also records the matched Native AOT image-size comparison; the one-off comparison apps are not included.

The prototype is not intended for production use or packaging.

make test passed, as did the benchmark project's --validate-json output and round-trip checks for all nine fixtures.

[codecov]

Codecov Report

❌ Patch coverage is 94.85000% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.46%. Comparing base (984ba23) to head (9e20030).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...lyType.FastPathJson/Converters/ScalarConverters.cs	82.59%	41 Missing and 14 partials ⚠️
...Type.FastPathJson.SourceGenerator/Parser/Parser.cs	93.93%	5 Missing and 16 partials ⚠️
.../PolyType.FastPathJson/Converters/EnumConverter.cs	93.95%	4 Missing and 5 partials ⚠️
...pe.FastPathJson/Converters/CollectionConverters.cs	93.44%	4 Missing ⚠️
...c/PolyType.Roslyn/Helpers/TypeIdentifierHelpers.cs	92.50%	3 Missing ⚠️
...SourceGenerator/SourceFormatter/SourceFormatter.cs	97.95%	0 Missing and 3 partials ⚠️
.../PolyType.FastPathJson/Converters/ScalarHelpers.cs	94.54%	2 Missing and 1 partial ⚠️
...hJson.SourceGenerator/Parser/DataModelExtractor.cs	98.18%	0 Missing and 2 partials ⚠️
...json/PolyType.FastPathJson/GenerationAttributes.cs	0.00%	2 Missing ⚠️
...rc/PolyType.Examples/Utilities/ByteBufferWriter.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   94.20%   94.46%   +0.25%     
==========================================
  Files         259      299      +40     
  Lines       17641    19556    +1915     
  Branches     3418     3813     +395     
==========================================
+ Hits        16619    18473    +1854     
- Misses        770      795      +25     
- Partials      252      288      +36     

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