refactor: move snapshot creation to historyStore (architectural fix)

Fixes architectural issue where snapshot logic was duplicated between
hook and store layers.

Problem Identified:
- useDocumentHistory (hook) created snapshots with duplicate logic
- timelineStore (store) created snapshots with duplicate logic
- timelineStore couldn't call useDocumentHistory (hooks can't be used in stores)
- Snapshot creation logic scattered across codebase

Root Cause:
Timeline operations happen in the store layer but history was managed
via a hook. This architectural mismatch forced duplication.

Solution:
- Add pushToHistory() method to historyStore
- Single source of truth for snapshot creation
- Both hook and store layers call historyStore.pushToHistory()
- Eliminates all snapshot creation duplication

Changes:
- Add historyStore.pushToHistory() - high-level API with snapshot creation
- Keep historyStore.pushAction() - low-level API (for future use)
- Update useDocumentHistory to call historyStore.pushToHistory()
- Update timelineStore pushDocumentHistory() to call historyStore.pushToHistory()
- Remove createDocumentSnapshot import from useDocumentHistory
- Remove createDocumentSnapshot import from timelineStore

Architecture:
Before:
  Hook (useDocumentHistory) → creates snapshot → historyStore.pushAction()
  Store (timelineStore) → creates snapshot → historyStore.pushAction()
  [Duplication at 2 call sites]

After:
  Hook (useDocumentHistory) → historyStore.pushToHistory() → creates snapshot
  Store (timelineStore) → historyStore.pushToHistory() → creates snapshot
  [Single implementation in historyStore]

Benefits:
- ✅ Zero snapshot creation duplication
- ✅ Proper architectural separation (store handles snapshots, not hook/store)
- ✅ Single source of truth (historyStore.pushToHistory)
- ✅ Timeline and graph operations use identical snapshot logic
- ✅ Easy to modify snapshot behavior in future (one place)

Impact:
- No breaking changes
- No behavior changes
- Better code organization
- Easier maintenance

Related: Phase 2.1 architectural improvement

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/hooks/useDocumentHistory.ts

@@ -4,7 +4,6 @@ import { useWorkspaceStore } from '../stores/workspaceStore';
 import { useHistoryStore } from '../stores/historyStore';
 import { useGraphStore } from '../stores/graphStore';
 import { useTimelineStore } from '../stores/timelineStore';
-import { createDocumentSnapshot } from '../stores/workspace/documentUtils';
 import type { DocumentSnapshot } from '../stores/historyStore';
 
 /**
@@ -59,25 +58,14 @@ export function useDocumentHistory() {
         return;
       }
 
-      // ✅ Use centralized snapshot creation (single source of truth)
-      const snapshot = createDocumentSnapshot(
+      // ✅ Call historyStore's high-level pushToHistory (single source of truth)
+      historyStore.pushToHistory(
         activeDocumentId,
+        description,
         activeDoc,
         timeline,
         graphStore
       );
-
-      if (!snapshot) {
-        console.warn('Failed to create snapshot');
-        return;
-      }
-
-      // Push to history
-      historyStore.pushAction(activeDocumentId, {
-        description,
-        timestamp: Date.now(),
-        documentState: snapshot,
-      });
     },
     [activeDocumentId, historyStore]
   );

src/stores/historyStore.ts

@@ -1,6 +1,9 @@
 import { create } from "zustand";
 import type { NodeTypeConfig, EdgeTypeConfig, LabelConfig } from "../types";
-import type { ConstellationState, StateId } from "../types/timeline";
+import type { ConstellationState, StateId, Timeline } from "../types/timeline";
+import type { Actor, Relation, Group } from "../types";
+import type { ConstellationDocument } from "./persistence/types";
+import { createDocumentSnapshot } from "./workspace/documentUtils";
 
 /**
  * History Store - Per-Document Undo/Redo System
@@ -52,9 +55,18 @@ interface HistoryActions {
   // Initialize history for a document
   initializeHistory: (documentId: string) => void;
 
-  // Push a new action onto the document's history stack
+  // Push a new action onto the document's history stack (low-level)
   pushAction: (documentId: string, action: HistoryAction) => void;
 
+  // Push action with automatic snapshot creation (high-level - USE THIS)
+  pushToHistory: (
+    documentId: string,
+    description: string,
+    document: ConstellationDocument,
+    timeline: Timeline,
+    graphStore: { nodes: Actor[]; edges: Relation[]; groups: Group[] }
+  ) => void;
+
   // Undo the last action for a specific document
   undo: (documentId: string, currentSnapshot: DocumentSnapshot) => DocumentSnapshot | null;
 
@@ -163,6 +175,34 @@ export const useHistoryStore = create<HistoryStore & HistoryActions>(
       });
     },
 
+    pushToHistory: (
+      documentId: string,
+      description: string,
+      document: ConstellationDocument,
+      timeline: Timeline,
+      graphStore: { nodes: Actor[]; edges: Relation[]; groups: Group[] }
+    ) => {
+      // ✅ Use centralized snapshot creation (single source of truth)
+      const snapshot = createDocumentSnapshot(
+        documentId,
+        document,
+        timeline,
+        graphStore
+      );
+
+      if (!snapshot) {
+        console.warn('Failed to create snapshot for history');
+        return;
+      }
+
+      // Call low-level pushAction
+      get().pushAction(documentId, {
+        description,
+        timestamp: Date.now(),
+        documentState: snapshot,
+      });
+    },
+
     undo: (documentId: string, currentSnapshot: DocumentSnapshot) => {
       const state = get();
       const history = state.histories.get(documentId);

src/stores/timelineStore.ts

@@ -11,7 +11,6 @@ import { useGraphStore } from "./graphStore";
 import { useWorkspaceStore } from "./workspaceStore";
 import { useToastStore } from "./toastStore";
 import { useHistoryStore } from "./historyStore";
-import { createDocumentSnapshot } from "./workspace/documentUtils";
 
 /**
  * Timeline Store
@@ -33,7 +32,13 @@ function generateStateId(): StateId {
   return `state_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
 }
 
-// Helper to push document state to history
+/**
+ * Helper to push document state to history from timeline operations
+ *
+ * This is a convenience wrapper that gathers the required state and calls
+ * historyStore.pushToHistory(). Timeline operations use this to track changes
+ * in the unified document history system.
+ */
 function pushDocumentHistory(documentId: string, description: string) {
   const historyStore = useHistoryStore.getState();
   const timelineStore = useTimelineStore.getState();
@@ -48,24 +53,14 @@ function pushDocumentHistory(documentId: string, description: string) {
     return;
   }
 
-  // ✅ Use centralized snapshot creation (single source of truth)
-  const snapshot = createDocumentSnapshot(
+  // ✅ Call historyStore's high-level pushToHistory (single source of truth for snapshots)
+  historyStore.pushToHistory(
     documentId,
+    description,
     document,
     timeline,
     graphStore
   );
-
-  if (!snapshot) {
-    console.warn('Failed to create snapshot');
-    return;
-  }
-
-  historyStore.pushAction(documentId, {
-    description,
-    timestamp: Date.now(),
-    documentState: snapshot,
-  });
 }
 
 export const useTimelineStore = create<TimelineStore & TimelineActions>(