pimdir move supplies a target placeholder

Following the io-replica staged-delete-and-move fix (a client-staged Remove or Move was a silent no-op in the hub), ReplicaMutation::Move gained a placeholder field mirroring Copy. The pimdir backend's move_messages now supplies it (move:<target>:<handle>), so a move stages a target create plus a source tombstone that the next sync propagates.

This is a mechanical caller update tracking the engine API; no backends requirement wording changed (the spec already states move stages a Move mutation the sync propagates). Behaviour fixed for users: message move — and message delete's trash-first move step, once mailbox.alias.trash is set — now take effect against a synced pimdir store instead of silently doing nothing. Verified live over the Fastmail-synced store (--account local): a move leaves the source and appears in the target, keeping the message's public id.

Capability touched: backends (no requirement change; caller-side fix logged for honesty).