Add observer track PNG movie mode

README.md

@@ -24,6 +24,31 @@ color.
 The output is a binary PPM at `output/imgs/minkowski_sky.ppm`; it can be inspected by
 most image viewers or converted to PNG with ImageMagick.
 
+## Movie PNG sequence (Phase A)
+
+Movie mode consumes a canonical observer-track CSV rather than a fixed camera.
+Each row stores coordinate time, proper time, Cartesian position, and the full
+four-by-four tetrad (21 columns total).  Generate the first reproducible
+Minkowski benchmark—two coordinate seconds at 30 fps, accelerating from rest
+to about `0.95c`—then render its numbered PNG frames:
+
+```sh
+make clean && make ENABLE_PNG=1
+mkdir -p output/imgs
+./build/minkowski_sky --write-minkowski-accel-track output/minkowski_accel_2s.csv \
+  --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
+```
+
+This writes `minkowski_accel_000000.png` through
+`minkowski_accel_000060.png`.  The renderer treats the CSV as its observer
+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.
+
 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: