feat(udp): SOCKS5 UDP ASSOCIATE relay through full tunnel

Adds end-to-end UDP support: SOCKS5 client UDP ASSOCIATE → tunnel-mux
udp_open/udp_data ops → tunnel-node UDP sessions → real UDP to upstream.
QUIC/HTTP3, DNS, and STUN now traverse full mode without falling back to
TCP or leaking outside the tunnel.

Apps Script proxies the new ops opaquely through the existing batch
endpoint; CodeFull.gs only gets a doc-comment update.

Highlights:
- proxy_server.rs: SOCKS5 UDP ASSOCIATE handler with per-session task,
  bounded uplink mpsc channel, adaptive empty-poll backoff (500 ms → 30 s),
  source-IP validation against the control TCP peer, port-locking on
  first valid datagram, and self-removal from the dispatch map on eof.
- tunnel_client.rs: UdpOpen / UdpData / close_session mux variants
  alongside the existing TCP plumbing; pkts decoder helper.
- tunnel-node: UdpSessionInner with bounded VecDeque queue, drop-oldest
  on overflow with queue_drops counter and warn-then-throttled logs,
  last_active refreshed only on real activity (uplink send or upstream
  recv — empty polls do not refresh), independent TCP/UDP drain in
  handle_batch Phase 2, separate active-drain (150 ms) and retry
  (250 ms) windows for UDP, idle long-poll (5 s).
- Tests: SOCKS5 UDP packet parser (IPv4/IPv6/DOMAIN round-trips,
  truncation rejects, fragmented rejects), UDP queue overflow drop +
  counter, regression test that batch with both UDP and TCP-data ops
  still runs the TCP retry pass.

Docs: README + android.{md,fa.md} updated to reflect UDP availability
in full mode; tunnel-node/README documents the new ops.

assets/apps_script/CodeFull.gs

@@ -6,6 +6,8 @@
  *   2. Batch relay:   POST { k, q: [{m,u,h,b,ct,r}, ...] }    → { q: [{s,h,b}, ...] }
  *   3. Tunnel:        POST { k, t, h, p, sid, d }              → { sid, d, eof }
  *   4. Tunnel batch:  POST { k, t:"batch", ops:[...] }         → { r: [...] }
+ *      Batch ops include TCP (`connect`, `data`) and UDP (`udp_open`,
+ *      `udp_data`) tunnel-node operations.
  *
  * CHANGE THESE TO YOUR OWN VALUES!
  */