libva-v4l2-request-ohm-gl-fix: mark deprecated, point at -fourier

Predecessor experimental package (tarball + 18 stacked patches) is
superseded by libva-v4l2-request-fourier (git-tracked fork tip,
iter38 multi-device probe). The successor already declares
replaces=('libva-v4l2-request-ohm-gl-fix') so installs migrate
automatically; this commit just makes the deprecation visible in
README + pkgdesc.

Kept in-tree as historical reference for the ohm_gl_fix Phase 6
audit trail.

arch/libva-v4l2-request-ohm-gl-fix/README.md

@@ -1,5 +1,24 @@
 # libva-v4l2-request-ohm-gl-fix
 
+> ## ⚠ DEPRECATED — use [`libva-v4l2-request-fourier`](../libva-v4l2-request-fourier/) instead
+>
+> This package is the **predecessor experimental** build (tarball pin
+> + 18 stacked patches) and is no longer maintained as of 2026-05-16.
+> Its successor `libva-v4l2-request-fourier` tracks the campaign fork's
+> git history directly
+> ([git.reauktion.de/marfrit/libva-v4l2-request-fourier](https://git.reauktion.de/marfrit/libva-v4l2-request-fourier))
+> so iteration sweeps (DEBUG removal, follow-up bugfixes) land in a clean
+> linear log, and adds the iter38 multi-device probe that lets a single
+> libva session serve rkvdec H.264/HEVC/VP9 + hantro MPEG-2/VP8 without
+> needing `LIBVA_V4L2_REQUEST_VIDEO_PATH` overrides.
+>
+> `libva-v4l2-request-fourier` declares
+> `replaces=('libva-v4l2-request-ohm-gl-fix')`, so installing it will
+> remove this package automatically. Kept in-tree as historical reference
+> for the ohm_gl_fix Phase 6 audit trail.
+
+---
+
 Bootlin's libva-v4l2-request VA-API backend, with hantro-vpu
 multi-planar + chromium-149-era stateless H.264 patches developed
 in the [ohm_gl_fix campaign](../../../ohm_gl_fix/) Phase 6 Step 1