[Draft] Use hybrid C# reference map for post-processing

[live1206]

Summary

Adds a hybrid reference-map replacement for C# generated-code post-processing.

The hybrid path replaces Roslyn reference-map construction with:

• provider metadata for generated-code references
• a small Roslyn scan only for custom/shared code roots

No benchmark measurement/profiling code is included in the production generator path in this PR.

Why

The prior experimental PR measured full generation and identified Roslyn reference-map construction inside GeneratedCodeWorkspace.PostProcessAsync() as the largest hotspot.

That experiment also showed the hybrid map can match Roslyn decisions for Sample-TypeSpec and improve local full-generation performance.

Latest measured data from the experimental PR:

Mode	Mean	Error	StdDev	Allocated
Baseline Roslyn reference-map path	1.042 s	0.363 s	0.418 s	63.98 MB
Hybrid provider/custom map replacement	641.3 ms	98.37 ms	113.3 ms	44.41 MB

Approximate improvement:

Time:       ~38.5% faster
Allocation: ~30.6% less

Correctness Check

For Sample-TypeSpec, the hybrid candidates matched Roslyn exactly in shadow comparison:

Internalize:
Roslyn candidates: 32
Provider candidates: 32
Missing: 0
Extra: 0

Remove:
Roslyn candidates: 37
Provider candidates: 37
Missing: 0
Extra: 0

Validation

• dotnet build packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Microsoft.TypeSpec.Generator.csproj -c Release passed.
• dotnet test packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Microsoft.TypeSpec.Generator.Tests.csproj --filter "FullyQualifiedName~GeneratedCodeWorkspaceTests" passed: 8/8.

Next Step

Run SDK service regeneration and compare generated output.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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