feat(core): ack send path with custody/broadcast buffers

Buffer pending acks (acks for addressed, bcast_acks for broadcast,
ACK_BUFFER_CAP=256, overflow drops oldest), skip a released-duplicate
offer in accept(), and emit Ack frames in next_frame after a due summary
and before addressed/broadcast traffic, draining up to ACK_IDS_CAP ids
per destination and splitting across frames to fit mtu.

Release custody on an Ack only when the Ack confirms delivery to the
intended recipient: an Ack from a peer that merely holds an
epidemic-forwarded copy no longer releases custody, so the local copy
stays available to forward toward the recipient once the only contact to
it closes.

tines-core/src/consts.rs

@@ -40,3 +40,8 @@ pub const EVENT_QUEUE_CAP: usize = 64;
 /// Cap on the `Exchange` per-peer `bcast_sent` table of broadcast ids
 /// dispatched to a peer.
 pub const BCAST_SENT_CAP: usize = 256;
+
+/// Cap on the number of pending ack ids (`acks` for addressed, `bcast_acks`
+/// for broadcast) buffered before the next Ack is emitted. Overflow drops
+/// the oldest pending entry.
+pub const ACK_BUFFER_CAP: usize = 256;