Output: default to PNG with PPM fallback

assets/2mass/processed/all_sky/README.md

@@ -38,7 +38,7 @@ tile tables after successful cleaning unless `--keep-raw` is passed.
 Use the directory mode rather than `--catalog` for this partitioned dataset:
 
 ```sh
-make ENABLE_PNG=1 SPACETIME=minkowski all
+make SPACETIME=minkowski all
 ./build/minkowski_sky --all-sky-catalog assets/2mass/processed/all_sky \
   --fov-deg 2 --look-ra-deg 180 --look-dec-deg -75 \
   --exposure 1e15 \
@@ -87,8 +87,9 @@ with the rotating observer fixture:
 Missing tiles are skipped until the downloader has produced them. Fully
 contained tiles skip per-star source-triangle containment; partial tiles retain
 that exact check. For 2MASS apparent-solid-angle amplitudes, start with
-`--exposure 1e15`; use PNG (`make ENABLE_PNG=1`) for substantially smaller
-single-frame output than binary PPM.
+`--exposure 1e15`; PNG is the default output format. If libpng is unavailable,
+use the PPM fallback with `make clean && make ENABLE_PNG=0` and a `.ppm`
+output path.
 
 The two existing fields imply roughly 15--17 GiB of cleaned CSV for the full
 PSC. Their raw response rows imply 70--75 GiB if every source appeared once.