feat: rebuild reactivity state from store after replay

Add `rebuildReactivityFromStore` to synchronize local reactivity
state with the hydrated variable store. This ensures tag activations
are correctly tracked following a reducer replay.

Also remove the unused `invalidateCompletions` function.

src/components/journal/var-reactivity.ts

@@ -167,6 +167,27 @@ export function setBase(key: string, value: string): void {
   baseValues.set(key, value);
 }
 
+/**
+ * Rebuild local reactivity state (baseValues, activeMods, sourceActivations)
+ * from the hydrated stream variable store. Must be called after replayReducers
+ * so that tag activations are correctly tracked for subsequent cascade
+ * computations.
+ */
+export function rebuildReactivityFromStore(variables: VariableStore): void {
+  if (!tagModMap) return;
+
+  for (const [key, value] of Object.entries(variables)) {
+    if (!baseValues.has(key)) {
+      baseValues.set(key, value);
+    }
+
+    const tag = isTagValue(value);
+    if (tag && !sourceActivations.has(key)) {
+      activateTagFromSource(key, tag, variables);
+    }
+  }
+}
+
 /**
  * Compute cascade effects after a variable change.
  *