fix(claude): keep a streamed reply on one message when message_start is lost

[Yevanchen]

Summary

• Anchor a stream scope to a synthetic burst key on the first content-bearing frame, so every frame of a streamed burst resolves to the same assistant message even when the message_start frame was lost. Previously each content_block_delta fell back to its own envelope uuid and minted a separate message.
• Let the aggregated assistant envelope consume an unconfirmed burst anchor (live or parked at message_stop) so it binds to the streamed message instead of re-emitting the full text as a new message; replayed envelopes stay bound via a native-id alias. Confirmed anchors from message_start keep today's semantics: an envelope with a different native id remains a distinct message, so final-message binding is unchanged.

Why

• YEF-884: a "ping" reply rendered as three assistant messages — 'P', 'ong. What would you like to work on?', and the duplicated full text — in both the session event log and the live view. Captured the real wire with Claude Code CLI 2.1.170 (--output-format stream-json --include-partial-messages): every envelope carries a fresh uuid (including each delta), and the aggregated assistant envelope arrives with the native message id before message_stop. Once message_start is missed (prewarm attach race), uuid-keyed identity fractures the reply exactly as reported. fix(claude): key assistant messages by API message id during streaming #52 keyed messages by the API message id when message_start is present; this closes the remaining fallback gap.
• The read-time repair for already-persisted rows lands separately in langgenius/mosoo (fold supersede); this change stops new fractured events at the source.

Verification

• bun test (1074 pass; the one failing ACP file-system test also fails on a clean checkout in this environment), bun run tc, bun run lint, bun run fmt.
• New regression test replays the captured YEF-884 wire shape (start-less per-uuid frames, envelope before message_stop) and asserts one message id across started/delta/snapshot/completed plus correct final-message binding.
• Updated the two degraded-stream tests that encoded uuid-as-identity: parallel tool blocks are isolated by content-block index within one burst, thought streams by message boundary — matching the real wire, where envelope uuids are per-frame and cannot identify a message.

Risk

• Behavior changes only for degraded streams (missing message_start); healthy streams resolve identically to today. Roll back by reverting the single commit.

[Yevanchen]

Clean-context review (agent, full production-evidence trace of session 01KY9Z5JEVPQP3F548XEK21MEG seq 36-43): APPROVE-WITH-NOTES.

• Mechanism confirmed: pre-PR, start-less stream_event frames fell back to per-frame envelope uuid → new messageId per frame → N× message.started + trailing full-text delta on a third message — exactly the production fracture shape. The anchor (anchorStreamingNativeMessageId on first content frame, alias-bound assistant envelope via resolveAssistantMessageNativeId) repairs identity at the root instead of adding a second dedup; frame-count-invariant so it covers N restarts.
• Trailing full-text duplicate: suppressed by the pre-existing hasStreamedText dedup once identity is unified; full text still lands as message.added snapshot on the same messageId (healthy-path parity).
• Edge cases checked: thinking blocks, mid-reply tool_use (toolCallId lookups now hit under degraded streams), two genuine messages not merged (confirmed-anchor semantics preserved + regression test), per-turn state reset.
• Notes (no change requested): (1) per-block envelopes on multi-block degraded bursts can split stream-frame parenting (text stays alias-bound); (2) a late burst-1 envelope after burst-2's first frame would merge bursts — impossible under the stated lost-prefix attach-race model. File as follow-ups only if telemetry shows multi-block degraded bursts.
• New transcript fixture replays the exact production topology; CI green; MERGEABLE/CLEAN, no rebase needed.
• Scope caveat: heals new events only; already-persisted fractured rows need the mosoo-side read-time fold (langgenius/mosoo#388 family).

Merging as-is.