compaction: the surface — settings row, two ruled dividers, summary dialog, send gate (round 14 slice 3)

- settings: compaction.thresholdTokens row on the Agent tab (owner pick)
  in a Context group — CommitInput mirroring the graph budget row, hint
  resolved through the SAME core resolver the trigger reads; PUT rejects
  below the 32k floor with the anti-churn rationale; own section
  (compaction) in SETTINGS_SECTIONS + the settings-signal union
- view fold: one marker → two dividers — the minimal MOMENT divider at
  the marker (retrying-message register, no rule lines) and the longer
  CUT divider emitted immediately before the first kept message
  (phase-divider geometry, chip carries '· Nk tok'); both carry the full
  summary payload; full history stays visible to the user
- components/compaction-divider.tsx: the dividers + summary dialog
  (pure exported body for SSR pins, the delete-dialog pattern; Esc +
  Close dismiss, backdrop does not)
- chat.tsx: compaction-changed SSE flips the running state — composer
  canSend, onSend/doSend/doSteerOrSend, and both docks' submit + hint
  ('compacting session…') gate on it (the server's registry mutex is the
  structural gate; this is the affordance); dialog mounts beside the
  abort dialog
- pins: settings-body (4), view fold (2), divider SSR (4)

app/api/settings/[section]/route.ts

@@ -5,7 +5,7 @@
  * discipline, same as the per-chat and defaults routes); sections are
  * independent, so partial writes can never clobber each other.
  *
- *   section ∈ general | chat | agent | appearance | graph (else 404).
+ *   section ∈ general | chat | agent | appearance | graph | compaction
  *   Body: the complete desired section object. Parse + validate strictly
  *   (server/settings-body.ts): a bad shape/type is a 400 carrying the
  *   supported list where one exists. The chat and models sections