firefox-fourier: add user-facing README explaining quick vs proper path

Two paths for HW decode through Firefox:
1. Easy: stock Firefox + MOZ_DISABLE_RDD_SANDBOX=1 (sandbox off,
   defense-in-depth lost — OK for personal-machine use)
2. Proper: apply 0001-rdd-allow-stateless-v4l2-request-api.patch to
   firefox-150.0.1 source, build (Arch overlay or generic mach build),
   install. Sandbox stays on; HW decode works.

Patch covers all three sandbox gates discovered iter3-5:
- Broker: cap-filter widening + /dev/media* enumeration
- Seccomp: ioctl magic byte '|' (linux/media.h)

README also points at the companion libva-v4l2-request-fourier repo
which carries the libva-side fixes (request_fd lifecycle, DPB
FFmpeg-semantics, B-slice L1, multi-context safety) needed alongside
the Firefox patch.

Top-level README cross-link added.

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

README.md

@@ -59,6 +59,8 @@ Reference history (context, NOT data this campaign anchors to) — orthogonal sc
 - [`~/src/x11-session-research/phase0_evidence/x11_baseline_2026-05-03/x1_summary.md`](../x11-session-research/phase0_evidence/x11_baseline_2026-05-03/x1_summary.md) — confirms the scanout-plane gap isn't fixable by switching session servers either. mpv-xv {SW,HW} and mpv-gpu {SW,HW} all leave Plane 39 in XRGB8888 throughout. It's a kernel/Mesa/Xorg-DDX gap, not a hardware-decoding gap. **Don't expect this campaign to "fix the video pipeline end-to-end"** — fixing libva-multi-planar fixes the decode side; the scanout-plane question stays open after.
 - [`~/src/kwin_overlay_subsurface/`](../kwin_overlay_subsurface/) — closed without patch (`phase8_handover.md`); its `feedback_replicate_baseline_first.md` lesson is the discipline that this campaign inherits.
 
+**Firefox patch** for the RDD sandbox: see [`firefox-fourier/README.md`](firefox-fourier/README.md). Two ways to get HW decode working — quick (env var, sandbox off) or proper (patched Firefox, sandbox kept on). Patch is ~50 lines, applies cleanly to firefox-150.0.1 source.
+
 External reference:
 
 - Mozilla bug 1833354 / 1965646 (Firefox HW decode on RK3566/RK3588 explicitly needs libva-v4l2-request, not v4l2-m2m).