The splice hazard the tail seed guards against is live in remote-lifecycle's
scrapeReadyPort: the remote spawn log merges stdout and stderr, yet it was
matched with the `^`-anchored READY_RE. Export one READY_IN_MERGED_OUTPUT_RE
(lookbehind token boundary — the hand-rolled `[^0-9A-Z_]` class admitted
lowercase-glued tokens) from backend-ready.ts and use it for both merged
buffers. Comments trimmed to the WHY; 6 new tests collapsed to the two
invariants (splice recovers, prose does not match) plus one remote-log case.
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