build: clear processed incremental changes

[ZluxYao]

Summary

• clear changesSinceLastBuild after a successful incremental rebuild
• clear the record in place to preserve references held by queued rebuilds
• add a regression test for the in-place state cleanup

Reproduction

After each successful incremental rebuild, previously processed changes remain in changesSinceLastBuild. Later rebuilds parse and emit those old changes again.

This causes the number of parsed files to accumulate across otherwise unrelated edits and can replay old delete events.

Testing

• npm run check
• npm test (164 tests passed)

This PR was written entirely using an LLM.

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[Copilot]

🟢 Approval recommended

The change is localized, addresses the stated incremental rebuild bug directly, and includes a focused regression test for the in-place cleanup behavior.

Pull request overview

This PR fixes incremental rebuild state management by ensuring changesSinceLastBuild is cleared after a successful rebuild, preventing previously processed change events from being re-parsed and re-emitted on later rebuilds. It introduces a small utility to clear the change record in place (preserving object identity for any queued rebuilds) and adds a regression test for that behavior.

Changes:

• Introduce clearChangeRecord() and ChangeRecord to standardize and safely clear the incremental change-tracking map in place.
• Clear changesSinceLastBuild after a successful rebuild to avoid replaying prior changes.
• Add a regression test ensuring the record is cleared without replacing the object reference.

File summaries

File	Description
quartz/util/incremental.ts	Adds ChangeRecord and an in-place clearChangeRecord() helper.
quartz/util/incremental.test.ts	Adds a regression test confirming the record is cleared in place.
quartz/build.ts	Uses ChangeRecord for changesSinceLastBuild and clears it after successful rebuild completion.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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