perf(net): own one pooled async receive

Replace timeout-polled in-world UDP receives with one cancellable caller-buffered socket operation. Transfer only right-sized pooled datagrams through the FIFO, return every ownership edge deterministically, and send caller spans without a transport copy while preserving handshake pacing and ACK order.

docs/plans/2026-07-25-modern-runtime-slice-h.md

@@ -135,6 +135,19 @@ behavior is outside this performance-only unit.
 
 ## 4. H-c — ordered, allocation-conscious network I/O
 
+H-c is executed as three independently reversible units:
+
+1. **H-c1 — pooled receive owner — COMPLETE.** One cancellable async socket
+   receive, a retained full-size edge buffer, right-sized pooled FIFO
+   datagrams, unchanged blocking handshake cadence, and direct span sends.
+   Evidence:
+   [`../research/2026-07-25-slice-h-c1-pooled-receive-owner.md`](../research/2026-07-25-slice-h-c1-pooled-receive-owner.md).
+2. **H-c2 — borrowed decode — ACTIVE.** Parse and verify headers, optional
+   fields, and fragments as borrowed views. Copy only multi-fragment state
+   that must survive the current datagram.
+3. **H-c3 — direct outbound framing — PENDING.** Write packet and fragment
+   framing into caller storage and remove intermediate payload arrays.
+
 Retail/transport invariants:
 
 - one outstanding receive;