feat(journal): seed declared variables on initialization

Implement `computeInitialValues` to calculate the starting state of
declared variables and seed them into the journal stream store during
the completions initialization process.

src/components/journal/var-reactivity.ts

@@ -174,6 +174,49 @@ export function computeCascade(
   return results;
 }
 
+/**
+ * Compute initial values for all declared variables.
+ * Called once after initReactivity() to seed the store.
+ * Unset dependencies default to 0.
+ */
+export function computeInitialValues(
+  currentVars: VariableStore,
+): Array<{ key: string; value: string }> {
+  if (!declExprs) return [];
+
+  const allKeys = [...declExprs.keys()];
+  const sorted = topoSortAffected(new Set(allKeys));
+
+  const results: Array<{ key: string; value: string }> = [];
+  const localVars = { ...currentVars };
+
+  for (const key of sorted) {
+    const expr = declExprs.get(key);
+    if (!expr) continue;
+
+    try {
+      const result = evaluateExpression(expr, {
+        lookup: (name: string) => {
+          const k = "$" + name;
+          return localVars[k] ?? undefined;
+        },
+      });
+
+      const newValue = String(result.value);
+      const finalValue = applyTagModifiersTo(key, newValue, localVars);
+
+      if (finalValue !== (localVars[key] ?? "")) {
+        localVars[key] = finalValue;
+        results.push({ key, value: finalValue });
+      }
+    } catch {
+      // skip failed evaluations at init time
+    }
+  }
+
+  return results;
+}
+
 /**
  * Compute which tags are active given the current variable store.
  * A tag is active if any variable has that tag as its value.