fix(core): keep bufferSeqNums in lock-step on session-stream on()-drain

`on()`'s buffer-drain block called `this.buffer.delete(key)` after
handing buffered records to the new handler but didn't delete the
parallel `this.bufferSeqNums` entry. The next `#dispatch` that buffered
a record would then append onto the orphaned seqNum array, and a
subsequent `once()` / `shiftBuffer()` would advance
`lastDispatchedSeqNum` with a stale seq — which would land on the next
`turn-complete` control record's `session-in-event-id` header and
cause the next worker boot to under-skip `.in`. Matches the cleanup
pattern already used in shiftBuffer / disconnectStream / reset.

Author: ericallam

packages/core/src/v3/sessionStreams/manager.ts

@@ -113,6 +113,10 @@ export class StandardSessionStreamManager implements SessionStreamManager {
         this.#invokeHandler(handler, data);
       }
       this.buffer.delete(key);
+      // Keep `bufferSeqNums` in lock-step with `buffer` — without this,
+      // the parallel array desyncs and the next `#dispatch` that buffers
+      // a record would shift a stale seqNum into `lastDispatchedSeqNum`.
+      this.bufferSeqNums.delete(key);
     }
 
     return {