release(v4.2.1): fix concurrent-ratchet desync via OutboundQueue waiter cursor

Pull-mode httpClient + drainer + parallel RPCs against the same peer
deteriorated after ~10s with `DecryptionError`. Two bugs combined:

- `OutboundQueue.enqueue` woke `drain` waiters with a `since=0`
  snapshot, replaying already-processed events into
  `Shade.acceptTransferEnvelope` → `manager.decrypt` twice. The
  duplicate consumed an already-used skipped key and corrupted the
  Double Ratchet receive chain.

- `ratchetDecrypt` then propagated the corruption: a same-DH
  message behind the chain with no cached skipped key fell through
  to `kdfChainKey` on the ahead state and rewound `chain.counter`,
  permanently desyncing the chain.

Fix `OutboundQueue` to honor each waiter's `since`, and harden
`ratchetDecrypt` so any future duplicate fails cleanly without
mutating state. Adds regression coverage at all three layers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shade-core/src/ratchet.ts

@@ -185,6 +185,22 @@ export async function ratchetDecrypt(
   if (!session.receiveChain) {
     throw new DecryptionError('No receiving chain available');
   }
+  // Defense-in-depth: a same-DH message whose counter is already
+  // behind the chain — and that did NOT match a cached skipped key —
+  // is either a duplicate we already decrypted (skipped key was
+  // consumed) or one whose key was evicted under cache pressure.
+  // Falling through would call kdfChainKey on the *current* (ahead)
+  // chainKey and then rewind `chain.counter = message.counter + 1`,
+  // permanently desyncing the chain so every subsequent decrypt
+  // returns wrong-key. Reject without mutating state instead.
+  if (
+    !isNewRatchet &&
+    message.counter < session.receiveChain.counter
+  ) {
+    throw new DecryptionError(
+      'Failed to decrypt message — wrong key or tampered data',
+    );
+  }
   await skipMessageKeys(crypto, session, message.dhPublicKey, session.receiveChain, message.counter);
 
   // Advance the receiving chain one more step to get this message's key