refactor: replace stat system with variable system

Replaces the specialized `stat` system with a more general-purpose
variable system. This includes:

- Renaming `ReactiveStatManager` to `ReactiveVariableManager`
- Changing template syntax from `${key}` to `{{$key}}`
- Replacing `StatsView` with `VariableView` to show declared, plain,
  and tag-typed variables
- Updating command `/stat` to `/set`
- Refactoring the reactivity engine to support variable declarations
  and tag modifiers via `csv role=declare` blocks

src/components/stores/journalStream.ts

@@ -44,7 +44,7 @@ export interface JournalStreamState {
   myRole: "gm" | "player" | "observer";
   brokerUrl: string | null;
   players: Record<string, { role: string }>;
-  stats: Record<string, string>;
+  variables: Record<string, string>;
 }
 
 export interface SessionMeta {
@@ -96,7 +96,7 @@ const [state, setState] = createStore<JournalStreamState>({
   myRole: (persisted.myRole as "gm" | "player" | "observer") || "gm",
   brokerUrl: persisted.brokerUrl,
   players: {},
-  stats: {},
+  variables: {},
 });
 
 if (initialName && !urlParams.playerName) syncUrlParam("player", initialName);
@@ -126,7 +126,7 @@ function runReducer(msg: StreamMessage): void {
 function replayReducers(): void {
   // Reset derived state
   setState("revealedPaths", {});
-  setState("stats", {});
+  setState("variables", {});
   for (const msg of state.messages) {
     runReducer(msg);
   }
@@ -160,7 +160,7 @@ function handlePatch(patch: WorkerPatch): void {
         syncUrlParam("player", patch.state.myName);
       }
       saveRole(patch.state.myRole);
-      // Rebuild derived state (revealedPaths, stats) by replaying reducers
+      // Rebuild derived state (revealedPaths, variables) by replaying reducers
       replayReducers();
       break;
     }