Catalog: parallelize all-sky tile prefetch

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

@@ -46,7 +46,44 @@ make ENABLE_PNG=1 SPACETIME=minkowski all
 ```
 
 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.
+It first collects and de-duplicates the frame's touched tile IDs on the main
+thread. Newly needed CSVs are then read by a bounded OpenMP loader group
+(`--catalog-load-workers N`, default `4`); each worker owns distinct tile
+files. The main thread commits their results to the cache only after that group
+finishes, so OpenMP splat workers always read immutable tile data and never
+race catalog I/O. A tile already loaded or previously found absent is not read
+again during the same renderer run. The cache deliberately remains live for
+the entire movie: final numerical-BBH renders commonly sweep across the full
+sky, so tile eviction would trade the normal case for repeated filesystem I/O.
+
+Each frame reports the number of requested/newly loaded/unavailable tiles,
+newly loaded stars, and CSV-load wall time. The loader count is deliberately
+independent of the splat worker count: start at the default and benchmark on
+the target storage before raising it.
+
+Warm-cache reference measurement, 2026-08-27, on the available partial
+all-sky directory (18 CSV tiles, 12,917 stars, 128x72, 2-degree field):
+
+| Loader workers | CSV load time |
+| ---: | ---: |
+| 1 | 5 ms |
+| 2 | 3 ms |
+| 4 | 2 ms |
+| 8 | 3 ms |
+
+This confirms the bounded-loader path and motivates the default of four; it is
+not a cold-cache throughput claim for the final full catalog. Reproduce it
+with the rotating observer fixture:
+
+```sh
+./build/minkowski_sky --all-sky-catalog assets/2mass/processed/all_sky \
+  --observer-track tests/data/minkowski_rotation_2s.csv \
+  --frames-dir output/imgs/catalog_prefetch_bench/w4 --frames-prefix frame \
+  --start-time 0 --duration 0 --fps 1 --width 128 --height 72 \
+  --coarse-cell-pixels 8 --fov-deg 2 --exposure 1e13 \
+  --catalog-load-workers 4
+```
+
 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