firefox-fourier: move vendor-prefs to defaults/preferences/ (the path Firefox actually scans)

Empirical follow-up to #9. After packaging 150.0.1-4 and installing on
fresnel, MOZ_LOG showed the failure pattern was still present:

  D/FFmpegVideo FFMPEG: Using preferred software codec h264

No VAAPI_VLD engagement, no dmabuf surface locking — Gecko bailed
before reaching the patched VAAPI path. Same symptom as issue #8
pre-fix, despite the prefs file being on disk at the expected path.

Root cause: /usr/lib/firefox-fourier/browser/defaults/preferences/ is
NOT a vendor-prefs scan location in Firefox 150. Mozilla's canonical
scan dir is /usr/lib/<app>/defaults/preferences/ — without the
'browser/' prefix.

Verified by hand-copying the same file to /usr/lib/firefox-fourier/
defaults/preferences/ and re-running the H.264 playback test:

  D/FFmpegVideo FFMPEG: Requesting pixel format VAAPI_VLD
  D/Dmabuf VideoFrameSurface: VAAPI locking dmabuf surface UID 26267
    FFMPEG ID 0x4000000 mAVHWFrameContext ...
  D/Dmabuf VideoFrameSurface: VAAPI locking dmabuf surface UID 26268
    FFMPEG ID 0x4000001 ...
  (15+ surface locks)

End-to-end zero-copy DMABUF path engaged, hantro/rkvdec dekodes the
H.264 stream via libva-v4l2-request-fourier iter38b.

pkgrel 4 -> 5. No other PKGBUILD changes.

arch/firefox-fourier/PKGBUILD

@@ -13,7 +13,7 @@
 
 pkgname=firefox-fourier
 pkgver=150.0.1
-pkgrel=4
+pkgrel=5
 pkgdesc='Firefox with V4L2 stateless HW video decode unlocked for mainline Linux Rockchip'
 arch=('aarch64' 'x86_64')
 url='https://www.mozilla.org/firefox'
@@ -171,6 +171,13 @@ LAUNCHER
   # at startup from the package install dir. The 0004 patch covers
   # media.ffmpeg.v4l2-request.enabled; this file covers the three
   # additional prefs that gate the path to the patched code.
+  # Vendor-prefs install path: /usr/lib/firefox-fourier/defaults/preferences/
+  # (Mozilla's canonical scan dir for third-party default-pref drops.) The
+  # browser/defaults/preferences/ alternative looked promising but is NOT a
+  # vendor-prefs scan location in Firefox 150 — empirically confirmed on
+  # fresnel: file shipped there, VAAPI never engaged. Same file under
+  # defaults/preferences/ → MOZ_LOG showed `Requesting pixel format
+  # VAAPI_VLD` + dmabuf surfaces locking end-to-end.
   install -Dm644 "${srcdir}/rockchip-fourier-defaults.js" \
-    "${pkgdir}/usr/lib/firefox-fourier/browser/defaults/preferences/rockchip-fourier-defaults.js"
+    "${pkgdir}/usr/lib/firefox-fourier/defaults/preferences/rockchip-fourier-defaults.js"
 }