fix(engine): prevent directScene hang + enforce segment ID uniqueness in prod

Two defensive fixes surfaced by the PR #95 review (PR-Agent), applied on
top of the staging sync:

1. directScene: routeTaggedStream rejecting BEFORE onPlan fires would leave
   planPromise unsettled, hanging `await planPromise` — and thus the whole
   /api/start and /api/scene request — forever. Add a .catch that settles
   the plan with a minimal fallback and resolves routing to a degraded
   result, so the pipeline produces a playable fallback scene (graceful
   degradation) instead of hanging.

2. prompts/registry: the duplicate-segment-ID guard only ran under
   NODE_ENV=development, so a bad merge introducing a duplicate ID would
   silently shadow a segment in production. Run the check in all
   environments (once at module load; negligible cost).

lib/engine/prompts/registry.ts

@@ -27,7 +27,11 @@ export const WRITER_SEGMENTS: PromptSegment[] = [
   WRITER_FORMAT,
 ];
 
-if (process.env.NODE_ENV === "development") {
+// Validate unique segment IDs in ALL environments (not just development).
+// A duplicate ID — e.g. introduced by a bad merge — would otherwise silently
+// shadow a segment in production. This runs once at module load; the cost is
+// negligible. Throwing fast surfaces the misconfiguration at startup.
+{
   const ids = WRITER_SEGMENTS.map((s) => s.id);
   const seen = new Set<string>();
   for (const id of ids) {