Class PendingResultQueue<T>

Ordered result queue for parallel-then-emit patterns.

Used by AsyncConvertTransfer and AsyncConditionTransfer when maxConcurrency > 1. Multiple async operations run in parallel, but their results are emitted to subscribers strictly in arrival order: a result with a higher sequence number waits in the queue until all lower-numbered results have been emitted.

Mechanics:

• nextSeq() — assigns a monotonically increasing sequence number
• submit(seq, value) — stores a completed result
• drain(handler) — emits all consecutively-numbered results starting from the expected sequence number
• clear() — discards all pending results (used on destroy)