refactor(net): type listener table as mapped type, drop double-casts

The listener map was typed as Map<GameClientEventType, AnyListener[]>, which
erased the per-type Listener<T> relationship and forced `as unknown as
AnyListener` double-casts at every on()/off()/emit() site.

Replace with a mapped-type record `{ [T in GameClientEventType]?: Listener<T>[] }`.
TS index lookup preserves the per-key relationship, so:

- off() has no cast
- emit() becomes generic over T and dispatches without casts
- on() retains a single scoped `Record<T, …>` projection at the write site
  (TS can't prove writes to a mapped-type index are safe under a generic T;
  this is a known limitation and the smallest workaround)

Also drops the now-unused LifecycleConnected/LifecycleDisconnected interfaces
(they existed only as emit() overload signatures, no longer needed with the
generic emit).

packages/chess/src/net/client.ts

@@ -59,20 +59,13 @@ type EventOfType<T extends GameClientEventType> = Extract<
 
 type Listener<T extends GameClientEventType> = (event: EventOfType<T>) => void;
 
-// Heterogeneous internal listener map — narrowed via the public `on()` API.
-// Using `unknown` avoids `any` while still permitting one map for all types.
-type AnyListener = (event: GameClientEvent) => void;
-
-// Shape emitted to listeners of a lifecycle-only event. We accept these two
-// shapes when callers invoke `emit()` so the compiler stays honest about the
-// discriminated union without resorting to casts.
-interface LifecycleConnected {
-  type: "connected";
-}
-interface LifecycleDisconnected {
-  type: "disconnected";
-  willReconnect: boolean;
-}
+// Heterogeneous internal listener table. Typing as a mapped type keyed by
+// the discriminant preserves the per-event Listener<T> relationship through
+// index lookup, so `on`/`off`/`emit` can manipulate listener arrays without
+// any casts.
+type ListenerTable = {
+  [T in GameClientEventType]?: Listener<T>[];
+};
 
 // ---------------------------------------------------------------------------
 // Configuration
@@ -142,8 +135,9 @@ export class GameClient {
   // `closed` is set when close() is called: it suppresses auto-reconnect.
   private closed = false;
 
-  // Listeners keyed by event type.
-  private readonly listeners = new Map<GameClientEventType, AnyListener[]>();
+  // Listeners keyed by event type. Mapped-type keys preserve the
+  // per-type Listener<T> relationship; see `ListenerTable`.
+  private readonly listeners: ListenerTable = {};
 
   constructor(url: string, options: GameClientOptions = {}) {
     this.url = url;
@@ -175,20 +169,21 @@ export class GameClient {
   // -------------------------------------------------------------------------
 
   on<T extends GameClientEventType>(type: T, listener: Listener<T>): void {
-    const arr = this.listeners.get(type);
-    // We up-cast to AnyListener here because the map is heterogeneous; the
-    // public `on` signature guarantees each listener only ever receives its
-    // own discriminated variant, which we enforce at `emit()` sites.
-    const cast = listener as unknown as AnyListener;
-    if (arr) arr.push(cast);
-    else this.listeners.set(type, [cast]);
+    // TS can't prove writes to `this.listeners[type]` are safe for a
+    // generic T (the mapped-type key makes the target an intersection).
+    // Projecting to a per-call `Record<T, …>` narrows the write site
+    // safely — this is the only place that widening happens, and it
+    // preserves the per-type Listener<T> relationship elsewhere.
+    const table = this.listeners as Record<T, Listener<T>[] | undefined>;
+    const arr = table[type];
+    if (arr) arr.push(listener);
+    else table[type] = [listener];
   }
 
   off<T extends GameClientEventType>(type: T, listener: Listener<T>): void {
-    const arr = this.listeners.get(type);
+    const arr = this.listeners[type];
     if (!arr) return;
-    const cast = listener as unknown as AnyListener;
-    const idx = arr.indexOf(cast);
+    const idx = arr.indexOf(listener);
     if (idx >= 0) arr.splice(idx, 1);
   }
 
@@ -473,11 +468,11 @@ export class GameClient {
     }
   }
 
-  private emit(event: GameClientEvent): void;
-  private emit(event: LifecycleConnected): void;
-  private emit(event: LifecycleDisconnected): void;
-  private emit(event: GameClientEvent): void {
-    const arr = this.listeners.get(event.type);
+  private emit<T extends GameClientEventType>(event: EventOfType<T>): void {
+    // Generic over T so `this.listeners[event.type]` resolves to
+    // `Listener<T>[] | undefined` (not a union), which matches the
+    // concrete `event: EventOfType<T>` being dispatched. No casts needed.
+    const arr = this.listeners[event.type];
     if (!arr) return;
     // Iterate over a copy so listeners that call `off()` mid-dispatch don't
     // skip subsequent listeners.