Feat: add initial HIP PSF backend

README.md

@@ -23,6 +23,26 @@ equator to the northern hemisphere, so boundary stars have a deterministic
 color.
 The default output is PNG at `output/imgs/minkowski_sky.png`.
 
+### Optional HIP PSF backend
+
+The default `PSF_BACKEND=cpu` uses the established OpenMP/private-HDR path.
+Build the direct-atomic HIP PSF backend explicitly with
+`PSF_BACKEND=hip`; it requires HIP/ROCm and a usable GPU agent, and writes a
+separate `_hip` binary so it cannot overwrite the CPU renderer:
+
+```sh
+make PSF_BACKEND=hip SPACETIME=minkowski backend
+./build/Release/minkowski_sky_hip --catalog assets/sky_grid_5deg.csv \
+  --output output/imgs/minkowski_sky_hip.png
+```
+
+The HIP backend accelerates only cache-eligible PSF events. Existing direct
+fallbacks remain CPU reference evaluations, with an ordered HDR transfer before
+and after each fallback. A HIP initialization, upload, kernel, or download
+error terminates the render; it never switches to the CPU backend silently.
+Use `make PSF_BACKEND=hip SPACETIME=minkowski hip-psf-test` for the small
+GPU-vs-CPU cache-HDR regression.
+
 ## Movie PNG sequence (Phase A)
 
 Movie mode consumes a canonical observer-track CSV rather than a fixed camera.