fix(sim): content-hash media URLs + fade pool-landing clip swap

Two fixes to the simulator's media handling, both surfaced by the cosmos clip swap.

1. Permanent cache-bust via content hash. New `GET /api/media-versions` returns a
   short sha1 token per served file (clip bases + ring transitions + reverses),
   cached by (mtime, size) so a re-bake is picked up without a restart. The client
   fetches it at boot and appends `?v=<hash>` to every /media URL, so a clip
   re-baked under the same path (e.g. a re-sourced cosmos base) gets a fresh URL
   the browser can't serve stale — fixing the recurring stale-clip problem at the
   root (supersedes the `cache: "reload"` belt-and-suspenders, which stays).

2. Fade the pool-landing swap. The baked ring transition zooms into the scale
   PRIMARY, but the rotating pool picks a random member on landing — so a
   non-primary pick hard-cut from the primary to the chosen clip (e.g. coast
   birdrock -> surfgrass, the artifact the operator saw). On landing, when the pick
   differs from the primary, mask the swap behind a short fade through the existing
   #black overlay (CSS opacity transition); same-clip landings still do a plain
   (re)load. General across all pooled scales.

271 passed, 2 skipped (+2: media-version content-hash + absent-file omission).
app.js syntax-checked; endpoint verified live (29 files; token matches a manual
sha1 of the bytes).

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

tests/test_simulator_api.py

@@ -76,6 +76,30 @@ def test_clips_returns_the_manifest(client):
     assert data["clips"][0]["annotations"][0]["key"] == "detected.water"
 
 
+def test_media_versions_endpoint_shape(client):
+    resp = client.get("/api/media-versions")
+    assert resp.status_code == 200
+    assert isinstance(resp.json()["versions"], dict)
+
+
+def test_media_version_is_a_content_hash(tmp_path, monkeypatch):
+    import hashlib
+
+    import simulator.app as appmod
+
+    # Point the media root at a temp dir (the served dir is otherwise fixed).
+    monkeypatch.setattr(appmod, "MEDIA_DIR", tmp_path)
+    (tmp_path / "clip").mkdir()
+    f = tmp_path / "clip" / "base.mp4"
+    f.write_bytes(b"hello-bytes")
+    assert appmod._media_version("clip/base.mp4") == hashlib.sha1(b"hello-bytes").hexdigest()[:12]
+    # Absent file -> None (it's simply omitted from the versions map).
+    assert appmod._media_version("clip/missing.mp4") is None
+    # Re-baked under the same path (new bytes) -> a new token, not the cached one.
+    f.write_bytes(b"different-bytes-entirely")
+    assert appmod._media_version("clip/base.mp4") == hashlib.sha1(b"different-bytes-entirely").hexdigest()[:12]
+
+
 def test_retired_selection_endpoints_are_gone(client):
     # The route no longer exists; the static catch-all yields 404 on GET and
     # 405 on the (now-unrouted) POST. Either proves the endpoint is gone.