Feat: Add dummy PSF chunk diagnostics

Reuse production catalog mapping and per-worker chunk boundaries to report triangle and spatial distributions without initializing HIP or writing image outputs. Include a protected full-catalog runner and build documentation.

build.md

@@ -104,6 +104,29 @@ times must not be added to GPU timings or interpreted as process CPU time.
 The two upload slots bound staging memory and allow CPU production ahead of GPU
 completion; copies and kernels still execute in order on one stream.
 
+### Diagnostic dummy PSF backend
+
+To inspect production chunk geometry without starting HIP or allocating an HDR
+framebuffer, build the diagnostic-only dummy backend:
+
+```sh
+make -j1 PSF_BACKEND=dummy SPACETIME=schwarzschild ENABLE_HDR=1 backend
+OMP_NUM_THREADS=16 OMP_DYNAMIC=FALSE \
+  ./build/Release/schwarzschild_sky_dummy --lens-map-input MAP.grlens \
+  --all-sky-catalog CATALOG_DIR --output PROTECTED_OR_UNUSED.png [normal PSF options]
+```
+
+The dummy executable requires an imported lens map and the PSF cache; it rejects
+`--psf-direct`. It runs the real OpenMP triangle scheduling, catalog query,
+inverse mapping, redshift/color/flux preparation, and cache/direct/min-Y
+classification, but records chunk statistics instead of accumulating pixels.
+`--output` and `--hdr-output` are accepted only for command parity: neither PNG
+nor FITS is created or modified. Reports separate full 16,384-event chunks from
+the final partial chunk of each worker and include event-producing triangle
+counts, occupied 32-pixel center tiles, bounding-box diagonals, and successive
+triangle-centroid jumps. This is a scheduling diagnostic, not a rendering or
+backend-performance benchmark.
+
 With both Minkowski binaries built using `ENABLE_HDR=1`,
 `python3 tests/test_hip_renderer.py build/Release/minkowski_sky build/Release/minkowski_sky_hip`
 runs small CPU/HIP fallback and min-Y comparisons sequentially. Each renderer