perf(sim): preload all clips into memory for instant altitude swaps

Each altitude change reset the single <video> element's src and re-fetched
the scale's footage over the network, compounded by the dev no-store policy
applying to /media too — so every scale change paid a full fetch+decode.

- simulator/app.py: split the cache middleware — app shell/JS/CSS stay
  no-store (by-eye dev iteration), but /media is now cacheable
  (public, max-age, immutable) since the clips are static.
- simulator/static/app.js: preload every ring clip (19 base + 5 transition,
  ~350 MB) into in-memory blob object URLs at startup — non-blocking, the
  first scale plays immediately while the rest stream in, ordered current
  scale outward. mediaUrl() serves the cached blob when ready, else falls
  back to the network. A small progress chip self-removes when done.
- tests: assert app assets stay no-store and /media is cacheable.

269 passed, 2 skipped. Frontend verified by-eye by the operator (no headless
browser in repo yet).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/app.py

@@ -234,12 +234,18 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
         return {"ok": True, "clip": merged}
 
     @app.middleware("http")
-    async def _no_cache(request, call_next):
-        # Dev preview server: never let a browser serve a stale app.js/style.css.
-        # The whole point is by-eye iteration, so a plain refresh must always get
-        # the latest assets (the heuristic cache otherwise hides JS/CSS edits).
+    async def _cache_policy(request, call_next):
+        # Dev preview server: never let a browser serve a stale app.js/style.css —
+        # by-eye iteration needs a plain refresh to always get the latest assets.
+        # Media is the exception: the clips are static and large, so they must be
+        # browser-cacheable. The renderer also preloads them into in-memory blob
+        # URLs (static/app.js), but a cacheable header keeps the preload fetch and
+        # any fallback off the network on repeat loads — instant altitude swaps.
         response = await call_next(request)
-        response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
+        if request.url.path.startswith("/media/"):
+            response.headers["Cache-Control"] = "public, max-age=31536000, immutable"
+        else:
+            response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
         return response
 
     if MEDIA_DIR.exists():