producer: the explicit bind-policy gate + the builder-register pin (review-002 C-3, D-2, S-1, N-2)

- C-3: Socks5ProducerBuilder.bind_policy(bool) — the substrate-free
  entry's explicit BIND gate, defaulting to the accept seam's
  presence; seam presence is never the sole gate
- D-2: RegisterMode::ProducerBuilder + an integration test wiring the
  full builder through .register(...) serving all three commands
- S-1: run_datagram_stage's doc names the hot-Ok(None) residual
- N-2: wire.md §UDP ASSOCIATE carries the inline-send backpressure line

Verified: cargo test (352 green), clippy/fmt/doc clean, wasm checks clean.

docs/architecture/wire.md

@@ -165,6 +165,10 @@ the same channel:
   sender's address per the RFC's per-datagram source semantics) or
   composed (a lazy destination→direct-op-tunnel table, LRU-evicted;
   open cost ~0.8 ms measured, per-target ACL for free).
+  **Per-datagram `send` is awaited inline** (the stage interleaves
+  sends with reply drains); a slow egress applies backpressure to
+  the association, not to other channels — the batch-2
+  composed-egress author inherits this bound knowingly.
 
 ### BIND: the phase model extended (ADR-007)