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/tests/ratchet.test.ts

@@ -281,6 +281,41 @@ describe('Double Ratchet', () => {
 
       expect(ratchetDecrypt(crypto, bob, msg)).rejects.toThrow();
     });
+
+    /**
+     * Regression — the v4.2.0 OutboundQueue waiter-since bug delivered
+     * the same envelope twice to `manager.decrypt`. The first decrypt
+     * succeeded via a cached skipped key; the second one fell into the
+     * `message.counter < chain.counter` path with no skipped key
+     * available, advanced the chainKey ONCE and rewound `chain.counter`
+     * to `message.counter + 1`, leaving the ratchet permanently
+     * desynced. ratchetDecrypt now rejects without mutating state when
+     * a same-DH message is behind the chain and not in skippedKeys, so
+     * a downstream replay (transport bug, retry, etc.) cannot poison
+     * the session for everyone else.
+     */
+    test('same-DH stale message after consumed skipped key fails without corrupting state', async () => {
+      const { alice, bob } = await setupPair();
+
+      // Alice sends 3 messages on the same DH chain.
+      const m0 = await ratchetEncrypt(crypto, alice, enc.encode('m0'));
+      const m1 = await ratchetEncrypt(crypto, alice, enc.encode('m1'));
+      const m2 = await ratchetEncrypt(crypto, alice, enc.encode('m2'));
+
+      // Bob receives m1 first, caching m0's key. Then m0 (delivered
+      // via the cache). After this, m0's skipped key is consumed.
+      expect(dec.decode(await ratchetDecrypt(crypto, bob, m1))).toBe('m1');
+      expect(dec.decode(await ratchetDecrypt(crypto, bob, m0))).toBe('m0');
+
+      // Replay of m0: skippedKey is gone, chain.counter is past m0.
+      // Pre-fix: this would corrupt Bob's chain state; post-fix it
+      // throws cleanly.
+      await expect(ratchetDecrypt(crypto, bob, m0)).rejects.toThrow(DecryptionError);
+
+      // Bob can still decrypt the remaining valid message — chain
+      // state was NOT mutated by the rejected replay.
+      expect(dec.decode(await ratchetDecrypt(crypto, bob, m2))).toBe('m2');
+    });
   });
 
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