fix(engine): pin entry-beat roster to the plan in Phase B

The Painter composites exactly plan.entryActiveCharacters into the entry
frame (the same roster the Cinematographer framed). Phase B is told to
reuse that roster, but only the entry beat's id was code-enforced — so an
LLM slip could leave a character in the painted frame that the runtime
entry beat says isn't there. Pin activeCharacters onto the plan's entry
beat as a last line of defense, mirroring the existing id pin.

Speaker is intentionally left to the prompt: it's coupled to line/TTS, so
overwriting it could mis-attribute or orphan Phase B's dialogue.

Addresses Copilot review feedback on PR #27.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/engine/agents/writer.ts

@@ -497,6 +497,15 @@ export async function runWriterBeats(
     beats = renameBeatId(beats, beats[0]!.id, plan.entryBeatId);
   }
 
+  // 把入场 beat 的 roster 钉成 plan 的：画师合成进帧的正是
+  // plan.entryActiveCharacters，运行时入场 beat 必须显示同一批人（与上面钉
+  // id 同理）。speaker 故意不钉——它和 line/TTS 耦合，强行覆盖会错配台词。
+  const entryRoster =
+    plan.entryActiveCharacters.length > 0 ? plan.entryActiveCharacters : undefined;
+  beats = beats.map((b) =>
+    b.id === plan.entryBeatId ? { ...b, activeCharacters: entryRoster } : b,
+  );
+
   return {
     beats,
     storyStatePatch: coerceStoryStatePatch(parsed.storyStatePatch),