fix: harden resume/shutdown paths, tighten Intent-18 and PSK handshake handling

Client (Go) — resume correctness
- C1: completeResume now re-parks the stream when replay fails mid-conn-loss.
  parked was cleared before the replay loop, so the dying conn's teardown
  would start a second resumeLoop and the two loops could strand the stream
  with neither alive. Resume stats are counted only after the replay lands.
- C2: RST(ALREADY_BOUND) is retryable instead of terminating the loop. With
  C1 fixed there is never a genuine second attempt, so "already bound" means
  the hub still holds the stream on a half-open conn; the retry waits out
  that bind (bounded by the grace deadline, teardown on expiry) instead of
  returning and leaving the destination socket hung forever.

Client (Go) — shutdown semantics
- C3: Close() sets a closing flag and cancels an internal context; dialSession
  takes a ctx (DialContext + AfterFunc so shutdown aborts in-flight
  handshakes); the worker pool refuses new conns after closeAll (Allocate,
  background growth, cond waiters); serveControl's reconnect loop is gated by
  closing so Close works even when the caller's Start context is not
  cancelled; conn-loss teardown closes streams outright during shutdown
  instead of parking them for a reattach that will never come.

Client (Go) — hygiene
- C4: pingInterval() clamps at the single point a duration is derived, so a
  hand-built Config with PingIntervalMs <= 0 can no longer panic
  time.NewTicker (added DefaultPingIntervalMs).
- E6: shaperStall is sampled right after shaper.Acquire, before the socket
  write, so a hub that is not reading is no longer charged to the bandwidth
  cap in the stats.
- E7: stream log lines now carry conn%d/sid%d (leg.String()), making streams
  traceable across reattaches.
- P5: mirror constants IntentReserved/RegisterOk/RegisterErrPattern added;
  RegisterAck dispatch logs rejection reasons via the named codes.

Hub (Java) + PROTOCOL.md
- P3: Intent 18 replies with a Minecraft status-response packet
  ([Len: VarInt][0x00][JSON: String]) and closes (socket.end, so the write
  always lands) instead of closing silently; documented in PROTOCOL.md §2.
- P4: PSK address check is strict equality with the lowercase hex address;
  an uppercase/case-folded variant is now rejected per PROTOCOL.md §2.
- P7: PROTOCOL.md §5 SessionReady row lists its real fields
  (Flags/RecvWindow/ResumeGraceMs) instead of "(none)".

Verified: go vet, go test -race ./client/..., gradle test, full e2e suite
(twice), resume e2e 3x, plus live probes of the hub with the real client
codec (Intent-18 status reply, strict-lowercase PSK acceptance/rejection).%

PROTOCOL.md

@@ -74,6 +74,19 @@ The hub reads exactly one Handshake packet and dispatches on `Intent`:
 | `18`          | Reserved for redapricot management/status. Never matched against patterns. The reference hub replies with a status line and closes. |
 | anything else | **Player** connection. `ServerAddress` is matched against the registered **regex** PATTERNs (§5.1). |
 
+For `Intent == 18` the hub replies with a Minecraft status-response packet and
+closes — the same shape a player receives for a status query (Intent 1), so the
+port can be probed with ordinary tooling:
+
+```
+[Len: VarInt][PacketID 0x00][JSON: String]
+```
+
+The JSON is a minimal status payload, e.g.
+`{"description":{"text":"redapricot hub"},"version":{"name":"redapricot","protocol":767},"players":{"max":0,"online":0}}`.
+It is the one reply the hub sends in plaintext: Intent 18 never negotiates
+encryption or any other redapricot state.
+
 For `Intent == 17` the hub additionally requires
 `ServerAddress == lowercase_hex(SHA3-224(PSK))` — a 56-character hex string.
 This is the first (cheap) proof that the peer knows the PSK. A mismatch closes
@@ -211,7 +224,7 @@ Type : u8
 
 | Type   | Name           | Direction | Fields |
 |--------|----------------|-----------|--------|
-| `0x00` | SessionReady   | S → C     | *(none)* — the confirmation frame from §4 |
+| `0x00` | SessionReady   | S → C     | `Flags: VarInt`, `RecvWindow: VarInt`, `ResumeGraceMs: VarInt` (only when the hub accepted `STREAM_RESUME`) — the confirmation frame from §4 |
 | `0x01` | Register       | C → S     | `Pattern: String` |
 | `0x02` | Unregister     | C → S     | `Pattern: String` |
 | `0x03` | RegisterAck    | S → C     | `Pattern: String`, `Status: u8` (0 = ok, 1 = invalid pattern) |