Output: simplify HDR output naming

README.md

@@ -256,7 +256,10 @@ the ray counts before each slab is loaded.
 To preserve a render for later exposure and tone-mapping work, build the desired
 backend with `ENABLE_HDR=1`, for example `make SPACETIME=schwarzschild
 ENABLE_HDR=1`. This produces `build/Release/schwarzschild_sky`,
-which accepts `--hdr-output PATH`. It writes the pre-tone-mapping RGB
+which accepts `--hdr-output`.  This switch writes the HDR file next to the
+ordinary output, replacing its extension with `_HDR.fits`; for example,
+`--output output/imgs/ring.png --hdr-output` writes
+`output/imgs/ring_HDR.fits`.  It writes the pre-tone-mapping RGB
 framebuffer as a three-plane, 32-bit float FITS image. Values remain linear HDR
 at the renderer's arbitrary scale; no tone mapping or per-frame normalization
 is applied. The ordinary