feat(web,engine): portrait-orientation scene images for mobile full-bleed

Thread orientation (portrait|landscape) from client through API, engine,
and image gen. Portrait devices render 1024x1792 (9:16) full-bleed scenes;
desktop/landscape keeps 1792x1024 (16:9). Adds cover-aware click→image
coordinate mapping, session-locked orientation, a shared coerceOrientation
helper, and a choices overflow cap in portrait.

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

lib/engine/orchestrator.ts

@@ -12,6 +12,7 @@ import type {
   VisionRequest,
   VisionResponse,
 } from "@infiplot/types";
+import { coerceOrientation } from "@infiplot/types";
 import { runArchitect } from "./agents/architect";
 import { directInsertBeat, directScene } from "./director";
 import { synthesizeBeat } from "./voice";
@@ -48,6 +49,7 @@ export async function startSession(
     history: [],
     characters: [],
     styleReferenceImage: req.styleReferenceImage?.trim() || undefined,
+    orientation: coerceOrientation(req.orientation),
   };
 
   // Stage 0 — Architect: expand the terse world/style prompt into a story