Review round findings:
- Clear the streaming attempt before the request-error waterfall: a cancel
landing during recovery (typically the llm/retry backoff, after clients
reset the streamed rendering) must not resurrect the failed stream's
prefix. Provider failures commit nothing, now including that window.
- Record interrupted: true on the cancellation-finalized assistant/message.
The chat projection keeps the settled prefix classified as interrupted
(Stopped chip, restored web goldens), and request inspection leaves the
request uncompleted so the step boundary classifies it as before.
- Pin the recovery-window and retry-discard semantics with content-bearing
failed streams in cancel.spec; update the ACP late-end expectation to the
finalized-prefix transcript.
- Mention interruptedBlocks() in the assembler stream-lifecycle JSDoc.
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