arch/chromium-fourier: scaffold PKGBUILD + placeholder patch

Initial chromium-fourier shape on the chromium-builder@boltzmann LXD
container we provisioned today. Approach is the 7Ji-style "tarball +
system tools" pattern (no gclient/CIPD, the linux-arm64 dependency
binaries don't exist anyway) but stripped of the MPP/X11/panfork
specifics — chromium-fourier targets mainline kernel + Wayland +
panfrost/panthor + V4L2 stateless on /dev/video0, not the vendor
stack 7Ji's chromium-mpp targets.

PKGBUILD highlights:
- pkgver=147.0.7727.116 (current Chrome stable as of 2026-04-25)
- gn args: use_v4l2_codec=true, use_v4lplugin=true, use_linux_v4l2_only=true,
  use_vaapi=true. The first three are the magic that unlocks V4L2VDA on
  Linux non-ChromeOS without source patches; if they're sufficient on
  their own, the chromeos-pipeline-bypass patch stays a no-op.
- ffmpeg_branding="Chrome" + proprietary_codecs=true for H.264.
- enable_widevine=false, enable_nacl=false to keep the tree small.
- Currently development-shaped: prepare()/build() operate on a
  pre-extracted /build/chromium/src rather than makepkg-fetched
  source. Will switch to canonical source=(...tarball.xz) shape once
  the patches stabilise.

patches/chromeos-pipeline-bypass.patch is a placeholder; the actual
patch (if any) gets developed once we see what 7Ji's gn args do or
don't unlock for us.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

arch/chromium-fourier/patches/chromeos-pipeline-bypass.patch

@@ -0,0 +1,22 @@
+From: Markus Fritsche <mfritsche@reauktion.de>
+Subject: media/gpu: skip chromeos VideoDecoderPipeline on non-ChromeOS Linux
+
+Placeholder. The patch will be developed against the actual chromium
+147.0.7727.116 source tree on chromium-builder@boltzmann once the
+tarball is extracted and we can read the exact code paths in:
+
+  - media/gpu/chromeos/video_decoder_pipeline.cc (PickDecoderOutputFormat,
+    Initialize, ImageProcessor setup)
+  - media/gpu/vaapi/vaapi_video_decoder.cc (ApplyResolutionChangeWithScreenSizes,
+    line ~1219 where "failed Initialize()ing the frame pool" fires)
+
+The 7Ji-style gn args (`use_v4l2_codec=true use_v4lplugin=true
+use_linux_v4l2_only=true`) MAY be sufficient by themselves to route
+decode through the legacy V4L2VDA path entirely, bypassing the
+chromeos pipeline without source patches. First build will tell us;
+this file is the placeholder for the patch we'll need if it isn't.
+
+# Empty no-op patch. patch -p1 < this won't change anything.
+diff --git a/PLACEHOLDER b/PLACEHOLDER
+new file mode 100644
+index 0000000..e69de29