F14.3b — Finish guidance explicitly without changing artwork

[Flow-Fly]

Parent: #131. Depends on #316.

Add an always-available Finish guidance action with explicit confirmation.

Acceptance:

• Finishing is available at every coverage percentage and never triggers automatically.
• Confirmation explains that the numbered scaffold and restrictions will be removed.
• Finishing removes only the guided-session payload; painted pixels and undo/redo history remain unchanged.
• The result returns to the normal editor and resets to a useful fitted view without motion choreography.
• The removal is saved immediately; a failed save restores guidance and shows an error.
• CRT remains unchanged and optional.

[Flow-Fly]

Completed in #319 and verified by post-merge develop CI run 29087949495.