bluetooth: add Reassembly.forget to reap a handle on disconnection

forget releases a handle's partially received PDU, reaping a reassembly that will never complete because the link was torn down (an HCI Disconnection Complete for that handle). Without it a handle that starts a multi-fragment PDU and never finishes retains its buffer forever -- one leak per churned short-lived BLE link. Pure and immutable, it completes the reassembler's lifecycle alongside active_handles.

The pure Bluetooth host reaps structurally: it holds the reassembler in the per-connection record and drops that record on Disconnection Complete, so a fresh connection on a reused handle starts empty and no partial carries over. forget is the reap primitive for a driver that shares one reassembler across handles (the fuzz model), and hardens the reassembler as a standalone table.

Also threads the survivor's post-completion state in the disconnection-reap fuzz added by the previous commit: a `_` pattern discarded the reaped state, so the final reap-to-baseline check saw a stale entry once forget existed.