feat: track followed NPCs via socket for LLM narration priority

Adds follow-npc client event so the server knows which NPCs are being
watched, enabling priority LLM narration for followed entity interactions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/network/SocketServer.ts

@@ -1,6 +1,6 @@
 import { Server } from 'socket.io';
 import type http from 'http';
-import type { ClientEvents, ServerEvents, PlayerInput, Position, Movement, PlayerControlled, Appearance } from '@dflike/shared';
+import type { ClientEvents, ServerEvents, PlayerInput, Position, Movement, PlayerControlled, Appearance, EntityId } from '@dflike/shared';
 import { Direction, MAX_NPC_COUNT } from '@dflike/shared';
 import { generateRandomAppearance } from '../spawner/appearanceGenerator.js';
 import type { GameLoop } from '../game/GameLoop.js';
@@ -10,6 +10,7 @@ import { serializeWorldState, serializeStateUpdate } from './stateSerializer.js'
 export class SocketServer {
   private io: Server<ClientEvents, ServerEvents>;
   private gameLoop: GameLoop;
+  private followedEntities: Map<string, EntityId | null> = new Map();
 
   constructor(httpServer: http.Server, gameLoop: GameLoop) {
     this.io = new Server(httpServer, {
@@ -82,15 +83,30 @@ export class SocketServer {
         spawnNPC(world, map, target);
       });
 
+      // Handle follow-npc
+      socket.on('follow-npc', (data: { entityId: EntityId | null }) => {
+        this.followedEntities.set(socket.id, data.entityId);
+        this.rebuildFollowedEntityIds();
+      });
+
       // Handle disconnect
       socket.on('disconnect', () => {
         console.log(`Player disconnected: ${playerId}`);
+        this.followedEntities.delete(socket.id);
+        this.rebuildFollowedEntityIds();
         world.removeEntity(entity);
         this.io.emit('player-left', { playerId });
       });
     });
   }
 
+  private rebuildFollowedEntityIds(): void {
+    this.gameLoop.followedEntityIds.clear();
+    for (const [, entityId] of this.followedEntities) {
+      if (entityId !== null) this.gameLoop.followedEntityIds.add(entityId);
+    }
+  }
+
   private handleInput(playerId: string, entityId: number, input: PlayerInput): void {
     const world = this.gameLoop.world;
     const pc = world.getComponent<PlayerControlled>(entityId, 'playerControlled');