Strict Isolation: Each "scope" (a chapter, or an outtake passage) maintains its own isolated undo history.
The Speed Bump: Holding Ctrl+Z undoes actions until the moment the document was opened this session, then strictly stops. The user must release the key and press again to undo past the current session boundary.
Structural Undo:Ctrl+Z reverses structural actions. Undoing a "Split Chapter" act rejoins the text and removes the newly created chapter file in one step.
Place Navigation:Ctrl+Alt+Z / Ctrl+Alt+Y moves the user through the "Previous/Next Place You Wrote", tracking only scopes that were actually modified, not just read.