refactor: pass variable store explicitly to getCombined

Remove the mutable `streamFallback` global state and instead pass the
`VariableStore` as an explicit argument to `getCombined` and related
functions. This ensures correctness by avoiding reliance on mutable
module state during reactivity calculations.

src/components/journal/command-dispatcher.ts

@@ -140,9 +140,9 @@ function dispatchSet(
   }
 
   const key = p.key;
-  // Use getCombined for old value so tag transitions are detected correctly
-  // even when the variable has active mods.
-  const oldValue = getCombined(key);
+  // Use getCombined with stream fallback for old value so tag transitions
+  // are detected correctly even when the variable has active mods.
+  const oldValue = getCombined(key, ctx.variables);
   let newValue: string;
 
   try {
@@ -158,8 +158,8 @@ function dispatchSet(
       const result = evaluateExpression(p.expr, {
         lookup: (name: string) => {
           const k = "$" + name;
-          // Use getCombined for accurate values including active mods
-          return k === key ? undefined : getCombined(k);
+          // Use getCombined with stream fallback for accurate values
+          return k === key ? undefined : getCombined(k, ctx.variables);
         },
       });
       newValue = String(result.value);
@@ -192,9 +192,9 @@ function dispatchSet(
   }
 
   // Send the direct set last so cascade messages arrive first.
-  // Use getCombined so the stream store receives the correct value
-  // (base + active mods) rather than just the raw base.
-  const combinedValue = getCombined(key);
+  // Use getCombined with stream fallback so the stream store receives the
+  // correct value (base + active mods) rather than just the raw base.
+  const combinedValue = getCombined(key, ctx.variables);
   const r1 = sendMessage("var", { action: "set", key, value: combinedValue });
   const u1 = unwrap(r1);
   if (!u1.ok) return u1;