consumer: AssociateSession — the framed datagram stage + e2e suite

The UDP ASSOCIATE consumer session (POC #2's shape, consumer.md §The
associate session, ADR-003):

- `AssociateSession::associate` — open → no-auth → ASSOCIATE → reply;
  the sentinel BND.ADDR is received (`relay_addr()`, RFC fidelity)
  and never used for addressing (the handle is the channel). The
  ASSOCIATE DST.ADDR is the vanilla-mirror unspecified address.
- `send_datagram` — frames exactly once via the crate's wire
  `frame()` (the framing-once mirror; the u16 joint bound enforced
  at frame time). Takes `impl ToTargetAddr` (ADR-012 posture); a
  `ToTargetAddr for &TargetAddr` impl was added to the forked codec
  so a `TargetAddr` value survives across sends.
- `recv_datagram` — the incremental `DatagramReader` + batched-
  datagram pending queue, state persisting across calls (the POC's
  reader-state finding); `Ok(None)` on channel EOF; fail-loud on a
  stream end mid-frame and on undecodable frames (`WireError` → the
  forked `AddrError`/`UdpHeaderError` RFC classes — the consumer
  mirror of the producer stage's `UndecodableFrame` exit).
- `shutdown` — the half-close; the mux's implicit-EOF sentinel
  reaches the producer's stage as EOF (association lifetime =
  channel lifetime). `close()`/`Drop` own the teardown (sync reap +
  structural EOF, the CONNECT/BIND posture); teardown-at-birth on
  in-stream refusals; no `Clone` (compile-fail asserted).
- Typed error surface: open failures are `SocksSessionError::Open`
  (ADR-049); the missing-egress 0x07 and factory-failure codes
  surface from `associate` as `Rfc`.

E2E suite (`tests/consumer-associate.rs`, 8 tests over the harness):
round-trips (multiple in-order datagrams, one association, sentinel
relay address asserted), the split-datagram persistence test (a 60
KiB datagram reassembling across the call boundary + the pending-
queue drain — both halves of the persistence invariant), the empty-
payload round-trip (F-2, never EOF), shutdown → a fresh associate
succeeds immediately (the POC's test 4), Drop teardown + connection
health, the missing-egress refusal (typed 0x07, no phantom channel),
the egress-factory failure mapping, and the CONNECT+ASSOCIATE
regression guard (interleaved on one registration). Unit tests (4)
pin the reader internals over a duplex (`skip_auth`'s
handshake-free path).

Verification: cargo test 305 (7 suites, all green), clippy
--all-targets -D warnings, fmt, cargo doc --no-deps, and both
wasm32-unknown-unknown checks clean.

src/rfc1928/util/target_addr.rs

@@ -200,6 +200,12 @@ impl ToTargetAddr for TargetAddr {
     }
 }
 
+impl ToTargetAddr for &TargetAddr {
+    fn to_target_addr(&self) -> io::Result<TargetAddr> {
+        Ok((*self).clone())
+    }
+}
+
 #[derive(Debug)]
 pub enum Addr {
     V4([u8; 4]),