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-key-transparency/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@shade/key-transparency",
-  "version": "4.2.0",
+  "version": "4.2.1",
   "type": "module",
   "main": "src/index.ts",
   "types": "src/index.ts",