perf: per-schema granularity in incremental generator pipeline (#170)

[pedrosakuma]

Summary

• stop collecting all XML AdditionalFiles into a single generation unit before RegisterSourceOutput
• generate types/messages/dispatcher/validation per schema while keeping semantic-type registry and analyzer config combined per item
• emit runtime helpers in a separate namespace-deduplicated step and add a driver-level incremental caching test

Validation

• dotnet build
• dotnet test

Closes #170