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/settings-dialog.tsx

@@ -34,6 +34,7 @@ import {
 	CURRENT_SETTINGS_VERSION,
 	type AgentSettings,
 	type AppearanceSettings,
+	type CompactionSettings,
 	type GraphSettings,
 	type UserSettings,
 } from "@/src/core";
@@ -216,7 +217,7 @@ export function SettingsDialogBody({
 	 *  schema mistake caught by the validation surface). */
 	const write = useCallback(
 		async (
-			section: "general" | "agent" | "models" | "appearance" | "graph",
+			section: "general" | "agent" | "models" | "appearance" | "graph" | "compaction",
 			body: object,
 		) => {
 			setError(null);
@@ -379,12 +380,16 @@ export function SettingsDialogBody({
 										agent={record.agent}
 										models={record.models}
 										graph={record.graph}
+										compaction={record.compaction}
 										onModelsChange={(next: ModelsSettings) =>
 											void write("models", next)
 										}
 										onSectionChange={(next: AgentSettings) =>
 											void write("agent", next)
 										}
+										onCompactionChange={(next: CompactionSettings) =>
+											void write("compaction", next)
+										}
 									/>
 								</SettingsSection>
 							)}