prose(experiment): the spawn offer, the return leg, and the graceful lock

[leeovery]

PR3 of the experiment-phase stack, on #1071. Built fresh against design/experiment-phase.md (#1057); replaces the integration layer of the closed #1062.

What this is

The seam between the conversations and the laboratory:

• The spawn offer (workflow-shared/references/experiment-spawn.md, loaded identically by research and discussion): when a number is about to bear a decision, the session offers the laboratory — conversationally, never a script, and declining keeps the inline measurement as a legitimate exploratory sighting. On yes the spawn is recorded right there: experiment create --from {phase} (id + lock), the plain-terms problem.md with its provenance line and no design content — the spawning phase is the client at the laboratory door — then the now-or-later gate (render experiment-spawn-gate). "Now" pauses the conversation mid-phase with no closing ceremony and hands the user to the menu in fresh context (/clear + /workflow-start); "later" continues with conclusion blocked. Both roads end at the menu; the laboratory is always entered fresh — nothing crosses into another phase's processing skill mid-session, and the bridge never carries a spawn.
• The wiring: the four conversation surfaces (research epic + linear, discussion epic + linear) gain the offer as a lettered section in the register of their existing off-topic triggers. Session loops otherwise untouched; bugfix and quick-fix never see it.
• The return leg: reconcile-advisory.md gains the experiment branch — at the spawning conversation's next entry the register renders, each newly terminal report is read in full, verdicts land as evidence the conversation weighs and can override, an abandonment surfaces its reason with the waiting point back open. Serves both flag origins (the holder released at a conclude/abandon, and a completed discussion flagged by the one-hop).
• The graceful lock: the research and discussion conclusion flows check awaiting_experiments beside their triage checks and render experiment-wait-gate — blocker naming the open ids, guidance naming the ways out, pause/keep menu. The engine's completion refusal stays the backstop, never the UX.
• Engine surfaces this layer owns: experiment-spawn-gate (refuses an id the item holds no wait on — the gate follows the recorded spawn) and experiment-wait-gate (refuses when nothing is owed), pinned in the render-surfaces suite and driven in the simulation.

Also: both epic-menu invokers now gloss the experiment route's fourth positional ($3 = the record id).

Gates: npm test 2670/2670 · npm run test:cli green · npm run test:migrations green · npm run typecheck clean.

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