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>

tests/test_simulator_api.py

@@ -91,6 +91,32 @@ def test_index_is_served():
     assert "Alteration" in resp.text
 
 
+def test_app_assets_are_not_cached(client):
+    # Dev iteration relies on the browser never serving stale renderer code.
+    resp = client.get("/")
+    assert "no-store" in resp.headers.get("cache-control", "")
+
+
+def test_media_is_cacheable(tmp_path, monkeypatch):
+    # Media is static: it must be browser-cacheable so the preloader's fetch (and
+    # any fallback) doesn't re-hit the network on every altitude change.
+    import simulator.app as appmod
+
+    media = tmp_path / "media"
+    (media / "cosmos").mkdir(parents=True)
+    (media / "cosmos" / "base.mp4").write_bytes(b"\x00\x00\x00\x18ftyp")
+    monkeypatch.setattr(appmod, "MEDIA_DIR", media)
+    manifest = tmp_path / "manifest.json"
+    manifest.write_text(json.dumps({"clips": []}))
+
+    client = TestClient(create_app(manifest_path=manifest))
+    resp = client.get("/media/cosmos/base.mp4")
+    assert resp.status_code == 200
+    cc = resp.headers.get("cache-control", "")
+    assert "no-store" not in cc
+    assert "max-age" in cc
+
+
 @pytest.fixture
 def ring_manifest_path(tmp_path):
     p = tmp_path / "manifest.json"