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)

components/settings-signal.ts

@@ -20,7 +20,7 @@ export const SETTINGS_SAVED_EVENT = "lh:settings-saved";
 // "keys" joined the roster (zero-keys wave, 2026-09-25): keys-section
 // dispatches it after save AND delete — deletions used to be invisible to
 // the zero-keys pill and the setup dialog's auto-advance.
-export type SettingsSection = "general" | "keys" | "agent" | "models" | "appearance" | "graph";
+export type SettingsSection = "general" | "keys" | "agent" | "models" | "appearance" | "graph" | "compaction";
 
 /** Writers call this after the PUT resolves ok — never before (a failed
  *  write notifies nothing; the record is unchanged). */