Phase 0: browser X11-overlay inventory + mpv reference cell + tooling installs

Source-level verdict: no browser in the matrix has a code path
to hand NV12 to the X server for plane scanout. Chromium ozone-x11
wires StubOverlayManager (ozone_platform_x11.cc:262); Brave 147 +
chromium-fourier 149 inherit unchanged. Firefox WindowSurfaceX11
is RGB-only. The campaign's load-bearing hypothesis is structurally
weakened — what the X11 cells will measure for browsers is
KWin's per-frame RGB-recomposite cost, not the original
"force-GL-composite of NV12" framing. mpv --vo=xv becomes the
matrix's only direct test of the operator-supplied mechanism.

Matrix updated: 12 cells -> 16 cells (+8 mpv VO sub-points).
revert.log entries 1-5 capture all package + per-user state
mutations from this turn (measurement tools, openbox, XFCE +
xfwm4-no-comp pre-seed, firefox + AUR xtrace, XFCE rotation
+ touchscreen mapping); single SSH-driveable revert chain
returns ohm to its pre-campaign 1169-package state.

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

README.md

@@ -24,9 +24,13 @@ forced GL-composite. **The campaign's load-bearing hypothesis
 is that this plane-allocation freedom translates into
 measurable browser-video speedup.**
 
-The matrix is 3 browsers × 2 decode paths × 2 sessions = 12
-cells (some N/A where libva isn't supported). See
-`phase0_findings.md` for the full table.
+The matrix is (3 browsers + mpv 0.41 as a reference X11-overlay
+client) × 2 decode paths × 2 sessions = 16 cells (some N/A
+where libva isn't supported). The mpv cells are run twice each
+— once with `--vo=xv` (legacy XVideo overlay) and once with
+`--vo=gpu --gpu-context=x11` (modern DRI3 + XPresent + Mesa GL).
+See `phase0_findings.md` § "Experimental matrix" for the full
+table and the rationale for mpv as reference baseline.
 
 ## Campaign-contained data discipline