Get incremental mode closer to Strada

[jakebailey]

This was copilot produced from an internal repro. massive baseline change, but:

• Clean  --noEmit --incremental  builds now keep source  fileInfos  compact when signature equals version, like  tsc .
• During affected-file processing, tsgo tracks  oldSignatures  while shape signatures are being updated, so buildinfo serialization can represent the same in-progress state Strada does.
• Forced declaration-signature emit work triggered from semantic diagnostics is now accounted as Emit time, not Check time, matching  tsc ’s diagnostic accounting.
• The original “incremental check is much slower than clean check” symptom is mostly an accounting mismatch: the expensive work still happens on the first affected incremental run in both compilers, but Strada reports it under emit/print rather than check.