fix: keep loaded documents clean after normalization - #5097

Summary

• After load-time markdown normalization, rebaseline editor history so an unmodified opened file stays clean.
• Prevents the save/don't-save prompt when closing a tab that was never edited.

Fixes #5074

Test plan

• Open a markdown file, make no edits, close the tab — no save dialog.
• Edit then close — save dialog still appears.
• Unit coverage in source-mode-dirty.spec.ts for load-time normalization rebaseline.