Persist each lane's answer as it lands, not after the last one (#58)

[CryptoJones]

Fixes #58 — the "roundtable randomly dies" bug.

The problem

main() collected the fan-out with results = list(pool.map(...)) and wrote the transcript only afterwards. Every answer was held in memory until the last lane returned, so any interruption during the fan-out discarded all completed lanes — including paid http lanes that had already billed for tokens nobody would ever read.

It presents as a run that dies with zero output and no transcript, and it bites hardest when it costs most: a long brief with many lanes, where the slowest lane decides whether the other eleven survive. Observed tonight on a 7-lane, 92 KB brief — stopped mid-flight, 0 bytes out, no transcript, after every lane had been dispatched.

The change

• Claim the transcript path before the fan-out
• Drive it with as_completed and rewrite the transcript after every lane
• Write each result into the slot its lane was dispatched from, so ordering is preserved and a partial transcript reads exactly like a complete one, with null for lanes still out
• Atomic writes (.tmp then replace), so a kill during the write leaves the previous good transcript instead of a truncated one
• Partial transcripts carry "partial": true plus a note; the final write clears the marker and attaches the synthesis

Behaviour on a successful run is unchanged.

Verification

• kill -9 mid-run now leaves a transcript holding the two lanes that had answered, third slot null, partial: true. Before the change that kill left nothing.
• Completed run: no partial marker, synthesis and collisions attached, no stray .tmp.
• Suite green — 119 passed, 29 subtests.
• Verification-set items 1 and 2 re-run by hand: --list shows the correct route per lane with zero network calls; a full run has every lane answer with exit 0.

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[coderabbitai]

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