Memory files are now explicitly treated as immutable across the system. Edit and Write tools are blocked for memory paths with the message: "is not permitted — memories are immutable, so delete-and-recreate replaces in-place edits." In tiny memory mode, the restriction is phrased as: "is not permitted in tiny memory mode — memories are immutable, so delete via Bash rm and rewrite."
Evidence
Memory immutability enforcement (search for "memories are immutable")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.