Output: write HDR FITS with camera metadata

README.md

@@ -174,8 +174,15 @@ the ray counts before each slab is loaded.
 
 For PSF validation only, `make psf-hdr-test` builds
 `build/minkowski_psf_hdr_test`, a separate binary with a `--hdr-output PATH`
-option.  It writes the pre-tone-mapping RGB framebuffer as a 32-bit float PFM
-image; the ordinary binaries do not contain this option or writer.
+option.  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
+binaries do not contain this option or writer.
+
+The FITS header describes a synthetic 8640-by-5760, 36-by-24 mm full-frame
+sensor with 4.1667 um pixels.  Each render records its active centered crop and
+derives `FOCALLEN` from that render's width and horizontal `--fov-deg`; these
+camera fields support plate-solving workflows but do not calibrate flux.
 
 Pass `--draw-mesh` to alpha-composite image-plane triangle edges as
 one-pixel-wide 0.5 linear-gray diagnostic lines at 0.5 opacity. The line