Use lower default exposure for movie tests

README.md

@@ -39,7 +39,7 @@ mkdir -p output/imgs
   --duration 2 --fps 30 --proper-acceleration 1.52
 ./build/minkowski_sky --observer-track output/minkowski_accel_2s.csv \
   --frames-dir output/imgs --frames-prefix minkowski_accel \
-  --start-time 0 --duration 2 --fps 30
+  --start-time 0 --duration 2 --fps 30 --exposure 1e-5
 ```
 
 This writes `minkowski_accel_000000.png` through
@@ -47,7 +47,10 @@ This writes `minkowski_accel_000000.png` through
 input; the acceleration generator is only a reproducible flat-spacetime test
 fixture.  The current movie path still renders frames independently while the
 time-slab/RayPool scheduler is implemented next, so it must not yet be used
-as a performance measurement for numerical-relativity data.
+as a performance measurement for numerical-relativity data.  Movie mode uses
+the same `1e-5` exposure by default when `--exposure` is omitted; this differs
+from the legacy single-frame default so the accelerated test catalog does not
+clip to white.
 
 PNG output is optional so the default build has no `libpng` dependency. Build
 with `make ENABLE_PNG=1`, then select it with a `.png` output path: