test: coverage weak spots — datagram EOF, UDP take_halves framing, reverse pump_against, queue contracts

Closes the behavioral test gaps the 2026-09-09 coverage review found
(88.28% → 92.86% lines; details in tasks/tunnels/coverage-weak-spots.md):

- Datagram EOF paths (consumer.rs:377-382, 255) — previously
  zero-covered documented contracts: a clean stream EOF resolves
  `Ok(None)` (the pump-less session's shutdown signal), a mid-datagram
  EOF fails loud with `TruncatedDatagram` (OQ-TN-13), both over a new
  closable UDP echo dial (`closing_udp_echo_dial`, both shapes).
- UDP `take_halves` (consumer.rs:201-205) — pins the N-3 framing
  contract: taken halves carry raw `[len: u16 BE]` wire bytes, and
  the data-plane accessors fail `ChannelTaken` after the take.
- Reverse UDP `pump_against` (consumer.rs:289) — the framed side
  round-trips end-to-end (payload + the F-2 empty datagram `len=0`
  survives the pump), the N-11 boundary contract.
- `AcceptQueue::push` after close returns `Err(handle)`, closed pop
  resolves `None`, and the accept loop's queue-closed mapping ends
  the loop with the handler error (producer.rs:279-288, local
  accept_loop).
- `establishment_reason` wrapper (params.rs) — unit test over the
  typed surface (open_failed reason / non-establishment /
  AdoptFailed); plus the schema-gate preemption probe in
  producer_open_op (missing substrate is `INVALID_INPUT` before the
  establisher runs).
- `UdpHalf` write machinery (local/mod.rs) — four poll-driven tests:
  queue accept + flush drain to the wire + shutdown best-effort,
  repeated flush cycles, the unconnected-socket error mapping
  (deterministic EDESTADDRREQ), and the shutdown-drain-swallows-
  errors posture. Host probes recorded in the task doc: no UDP
  sender-blocking shape is reachable on this host (loopback/veth
  drop silently; the fresh-socket try_send quirk is timing-tolerated,
  never asserted), so the stash/high-water paths stay uncovered by
  host limitation, not by oversight.

Verified: cargo test (default + --all-features), clippy --all-targets
-D warnings, fmt --check, wasm32 check + clippy, llvm-cov 92.86%.

Cargo.toml

@@ -31,3 +31,4 @@ thiserror = "2"
 [dev-dependencies]
 tokio = { version = "1", features = ["full", "test-util", "macros"] }
 tracing-subscriber = { version = "0.3.23", features = ["env-filter"] }
+socket2 = "0.6"