Preserve concurrent page edits during rollback

[jzjzzzzzzz]

Summary

• record the verified published after-image state for each projected page
• make rollback compare the live canonical page with that exact after-image before removing it
• preserve concurrent bytes, the rollback image, writer lock, and recovery journal when the comparison fails
• retain the pre-verification rollback path for a corrupted stage that never became a verified publication
• add a regression test covering an in-place concurrent edit followed by a later-page failure

Testing

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest Tools.tests.test_project_page_state
• make check
• full local suite: 1087 tests pass; the existing 6 macOS /var vs /private/var failures in test_apply_task_plan remain unrelated and are addressed separately by fix/macos-repository-root-alias

Fixes #59.