Per-message HashMap allocs in session sender

[muscariello]

Files:

• core/session/src/session_sender.rs — on_publish_message()
• core/session/src/producer_buffer.rs — push()

Problem:
In reliable-session mode, every outbound message triggers:

1. producer_buffer.push() — HashMap::insert(msg_id, slot).
2. pending_acks.insert(msg_id, …) — another HashMap insert.
3. ack_notifiers.insert(msg_id, oneshot_tx) — a third HashMap insert.
4. A per-message retransmission timer (tokio::spawn of a timer task).

Three HashMap inserts + a spawned task per message is significant at high
rates.

Mitigation plan:

1. Replace the three per-message HashMaps with a single indexed structure
   (slab or ring buffer keyed by msg_id % window).
2. Use a timer wheel (e.g., tokio_util::time::DelayQueue) instead of
   spawning individual timer tasks. One task manages all pending timeouts.
3. Batch ACK processing: coalesce multiple ACKs before modifying data
   structures.