Make page copy roll back partial destination pages on failure

[Flerpharos]

Summary
A failed page copy should not leave a partially populated destination page behind.

Scope

• track destination page creation and appended entries
• clean up partial page state when copy fails midway
• define the failure invariant clearly in docs and tests

Acceptance criteria

• simulated mid-copy failures do not leave partial page content behind
• callers get consistent failure behavior
• tests cover rollback or cleanup on failure