Add lazy tiled 2MASS catalog support

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

@@ -33,6 +33,26 @@ as the sample processor. The downloader fails if an IRSA response reaches
 `--outrows`, so crowded fields cannot be silently truncated. It deletes raw
 tile tables after successful cleaning unless `--keep-raw` is passed.
 
+## Renderer use
+
+Use the directory mode rather than `--catalog` for this partitioned dataset:
+
+```sh
+make ENABLE_PNG=1 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 \
+  --output output/imgs/2mass_all_sky_flat_2deg.png
+```
+
+The renderer lazily loads only tiles touched by source triangles in the frame.
+It prefetches them serially, so OpenMP splat workers read immutable tile data.
+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.
+
 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.
 The cover grid uses substantially fewer cone areas than the old fixed-grid