Revert on_ack release gating to release-on-any-ack (custody chain): release
custody on the first exact Ack from ANY peer the node dispatched the
observation to (relay or recipient), not just the recipient.
drain_ack_buffer send_n==0 bug: re-keep only the target addr's own pulled
entries instead of removing buffer.entries[0] (which dropped an unrelated
peer's oldest pending ack).
Rename the tines-sim addressed proptest to
addressed_always_held_by_carrier_or_recipient and reconcile its assertion to
the custody-chain holder invariant. Reword the misleading "Ids shrink
monotonically" comment. Add unit tests: ack_releases_custody_from_a_relay and
drain_ack_buffer_overflow_drops_only_target_peer.
Gate green: tines-core 106 lib tests, tines-sim suite, sh scripts/ci.sh.
