feat(vibe): plan musical arcs and callbacks

backend/src/services/vibe-session-coordinator.service.ts

@@ -1,5 +1,6 @@
 import { DbService, VibeEvent, VibePlan, VibeSession } from './db.service.js';
 import { SessionDirector } from './session-director.service.js';
+import { Candidate } from './generators.service.js';
 
 /**
  * This is deliberately a narrow bridge between the durable Vibe ledger and
@@ -67,7 +68,7 @@ export interface VibeSessionResponse {
 export class VibeSessionCoordinator {
   constructor(
     private readonly db: DbService,
-    private readonly director: Pick<SessionDirector, 'buildPlan' | 'buildState'>,
+    private readonly director: Pick<SessionDirector, 'buildPlan' | 'buildState' | 'replan'>,
   ) {}
 
   async start(userId: string, input: StartVibeSessionInput): Promise<VibeSessionResponse> {
@@ -111,15 +112,18 @@ export class VibeSessionCoordinator {
           policyVersion,
           intent: input.intent ?? null,
           horizonTracks: candidates.length,
+          ...(candidates[0]?.plan?.objective ?? {}),
         },
         items: candidates.map((candidate, ordinal) => ({
           ordinal,
           track_id: candidate.trackId,
-          slot_role: ordinal === 0 ? 'next' : null,
+          slot_role: candidate.plan?.slotRole ?? (ordinal === 0 ? 'next' : null),
           candidate_source: candidate.generatorId,
-          score: candidate.relevance,
-          score_breakdown: { relevance: candidate.relevance },
-          explanation: candidate.explanation,
+          score: candidate.plan?.score ?? candidate.relevance,
+          score_breakdown: candidate.plan?.scoreBreakdown ?? { relevance: candidate.relevance },
+          explanation: candidate.plan
+            ? { paths: candidate.explanation, planner: candidate.plan.explanation }
+            : candidate.explanation,
           committed: false,
         })),
       });
@@ -216,11 +220,24 @@ export class VibeSessionCoordinator {
       // feedback tracks, the seed is not necessarily present in the event
       // ledger, so carry it explicitly into every replacement request.
       const seedTrackId = session.seed_track_id ?? undefined;
-      const candidates = seedTrackId
-        ? await this.director.buildPlan(userId, sessionId, input.trackId ?? seedTrackId, {
-          excludedTrackIds: new Set([seedTrackId]),
-        })
-        : await this.director.buildPlan(userId, sessionId, input.trackId);
+      // Revisions retain the durable, unserved queue tail rather than building
+      // an unrelated plan after every signal. Besides reducing churn, this
+      // preserves a valid callback/recovery pair that has already been shown
+      // to the client while allowing the director to refill under the same
+      // hard caps and current feedback state.
+      const current = await this.db.getVibePlan(sessionId, userId);
+      const retained = current ? this.unservedCandidates(current) : [];
+      const excludedTrackIds = new Set<string>([
+        ...(seedTrackId ? [seedTrackId] : []),
+      ]);
+      const candidates = await this.director.replan(
+        userId,
+        sessionId,
+        retained,
+        input.trackId ? [input.trackId] : [],
+        input.trackId ?? seedTrackId,
+        { excludedTrackIds },
+      );
       const reason = `feedback:${input.type}`;
       const plan = await this.db.publishVibePlan({
         sessionId,
@@ -232,15 +249,18 @@ export class VibeSessionCoordinator {
           feedbackEventId: result.event.id,
           feedbackType: input.type,
           horizonTracks: candidates.length,
+          ...(candidates[0]?.plan?.objective ?? {}),
         },
         items: candidates.map((candidate, ordinal) => ({
           ordinal,
           track_id: candidate.trackId,
-          slot_role: ordinal === 0 ? 'next' : null,
+          slot_role: candidate.plan?.slotRole ?? (ordinal === 0 ? 'next' : null),
           candidate_source: candidate.generatorId,
-          score: candidate.relevance,
-          score_breakdown: { relevance: candidate.relevance },
-          explanation: candidate.explanation,
+          score: candidate.plan?.score ?? candidate.relevance,
+          score_breakdown: candidate.plan?.scoreBreakdown ?? { relevance: candidate.relevance },
+          explanation: candidate.plan
+            ? { paths: candidate.explanation, planner: candidate.plan.explanation }
+            : candidate.explanation,
           committed: false,
         })),
       });
@@ -303,6 +323,37 @@ export class VibeSessionCoordinator {
     };
   }
 
+  /** Reconstruct the planner envelope from the durable revision. Older
+   * revisions stored only paths, so they remain valid retention inputs. */
+  private unservedCandidates(plan: VibePlan): Candidate[] {
+    return plan.items
+      .filter(item => !item.committed)
+      .map(item => {
+        const stored = item.explanation;
+        const hasPlanner = !!stored && !Array.isArray(stored) && typeof stored === 'object'
+          && 'planner' in stored;
+        const object = hasPlanner ? stored as { paths?: Candidate['explanation']; planner?: Record<string, unknown> } : undefined;
+        const planner = object?.planner;
+        return {
+          trackId: item.track_id,
+          generatorId: item.candidate_source,
+          relevance: item.score,
+          explanation: object?.paths ?? (Array.isArray(stored) ? stored : []),
+          plan: planner ? {
+            slotRole: item.slot_role ?? 'retained',
+            score: item.score,
+            scoreBreakdown: item.score_breakdown,
+            explanation: planner,
+            objective: {
+              policy: planner.policy,
+              constraints: planner.constraints,
+              relaxations: planner.relaxations,
+            },
+          } : undefined,
+        };
+      });
+  }
+
   private mapLifecycleError(error: unknown): Error {
     if (error instanceof Error && (error.message.includes('Cannot record a new event for') || error.message.includes('Cannot resume ') || error.message.includes('Cannot publish a plan for'))) {
       return new VibeSessionLifecycleError(error.message);