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>

worklist.md

@@ -52,21 +52,38 @@ inventory and baseline-anchor work below.
   sessions. Operator action: `pacman -S` a non-compositing WM
   (recommend openbox) and create/switch to its SDDM session
   before without-KWin cells can run.
-- [ ] **NEW: Browser X11-overlay-path inventory.** Per
-  `phase0_findings.md` § "Open questions": determine whether
-  Brave 147 ozone-x11, chromium-fourier 149 ozone-x11, and
-  Firefox X11 backends actually request hardware-overlay
-  presentation for windowed video, or whether they always
-  internally composite to RGB. Browser-specific source-grep
-  + chrome trace inspection.
-- [ ] **NEW: Add mpv as a reference X11-overlay client.**
-  mpv with `--vo=xv` or `--vo=gpu --gpu-context=x11` is a
-  known-good X11 hardware-overlay path. Adding mpv to the
-  matrix as a 4th client provides "is the X11 hardware-overlay
-  path even reachable on this hardware" baseline, separate
-  from "do browsers use it." If mpv hits Plane 39 NV12 cleanly
-  but browsers don't, the answer is "X11 path is fast, but
-  the browsers don't take advantage of it."
+- [x] **Browser X11-overlay-path inventory.** Captured
+  2026-05-03 in
+  `phase0_evidence/browser_overlay_inventory_2026-05-03.md`
+  + acquired-source subtree
+  `phase0_evidence/chromium_ozone_x11_2026-05-03/` (Chromium
+  147 from `chromium-builder` LXD CT on boltzmann via the
+  his subagent). **Decisive verdict at the source level:**
+  Chromium ozone-x11 instantiates `StubOverlayManager`
+  (`ozone_platform_x11.cc:262`) — no overlay candidates ever
+  promoted; Brave 147 + chromium-fourier 149 inherit this
+  unchanged (their patches don't touch Ozone). Firefox 150
+  `WindowSurfaceX11{,Image,SHM}` are RGB-only (zero NV12 /
+  dmabuf / DRI3 / XPresent references in any X11-surface
+  file). **No browser in the matrix has a code path to
+  hand NV12 to the X server for plane scanout.** Only mpv
+  `--vo=xv` does; mpv `--vo=gpu --gpu-context=x11` is the
+  same GL-composite shape as the browsers. Implication:
+  campaign's load-bearing hypothesis is structurally
+  weakened — the X11-vs-Wayland delta the matrix will
+  measure for browsers is "browser-side GL composite +
+  X-scanout-of-RGB" vs "browser-side GL composite +
+  KWin-RGB-recomposite + scanout". mpv-xv becomes the
+  matrix's only direct test of the original mechanism.
+  Runtime-engagement probe deferred to Phase 1 first rep
+  (design sketched in the inventory doc).
+- [x] **mpv added as 4th matrix client** 2026-05-03. Two cells
+  (full SW, libva/v4l2request) × 2 sessions × 2 VOs (`--vo=xv`
+  and `--vo=gpu --gpu-context=x11`) = 8 reference data points.
+  README + `phase0_findings.md` § "Experimental matrix"
+  updated; matrix grew from 12 → 16 cells (+8 mpv VO sub-points).
+  mpv 0.41.0 already installed via predecessor (see
+  `phase0_evidence/x11_inventory_2026-05-03/02_x11_paths.txt`).
 - [x] Inventory of X11-side measurement instruments. Captured
   2026-05-03 in `phase0_evidence/x11_inventory_2026-05-03/`
   (raw `04_measurement_instruments.txt` + summary `inventory.md`