fix: finalize source projection before unrelated document changes

[Azganoth]

Summary

• Finalize an unchanged source projection when an unrelated document change moves selection outside its range, instead of dropping the session and leaving source literal.
• Keep that external change in native history; the clean restore maps its undo and redo steps onto canonical content.
• Cover document, selection, serialization, dirty tracking, undo, and redo against an explicit-finalize baseline.

Related Issue

Closes #472

Verification

• pnpm exec vitest run src/features/editor/plugins/sourceProjection.test.ts — exercises source-projection behavior, including the new unrelated-change regression.
• pnpm check:frontend — checks formatting, TypeScript, Oxlint, production build, and frontend coverage.
• git diff --check — confirms no whitespace errors.

Not verified: manual desktop interaction or the desktop E2E suite.