display: pace the frame clock by the panel's EDID refresh rate

Both EDID moments the system has are now captured and carried to the
compositor's frame clock:

- EFI: the loader derives refresh from the preferred detailed timing
  (pixel clock / total pixels) while GOP is still alive — the only moment
  it is readable — and hands it through the boot handoff into the
  display0 node's DisplayInfo (new refresh_hz field, 0 = unknown).
- GPU: the virtio-gpu driver derives the same figure from its own EDID
  read and carries it in the attach_scanout announce (request.y).

updateFrameClock() re-derives the interval from the active backend's
info at bring-up and again on every backend change — the boot
framebuffer's clock dies with the GOP floor at upgrade, replaced by the
GPU's rate. Unknown rate defaults to 60 Hz; the result is clamped to
[30, 120] Hz so a mis-parsed EDID can neither starve nor flood the
compositor. Rate only, never phase: without vblank, presents still
free-run (docs/display-v2.md, 'Fenced is not vsync').

Observed in QEMU: OVMF exposes no EDID for the VGA adapter, so the GOP
floor logs 'frame clock 62 Hz (default)' (real firmware does expose it);
the virtio-gpu EDID advertises 75 Hz and the upgrade logs 'frame clock
76 Hz (panel EDID)'. The display kernel test asserts refresh_hz rides
the seeded node; all 7 display QEMU cases pass.

system/boot-handoff.zig

@@ -37,6 +37,11 @@ pub const Framebuffer = extern struct {
     height: u32, // visible rows (e.g. 1080)
     pitch: u32, // bytes from the start of one row to the start of the next
     format: PixelFormat,
+    /// The panel's refresh rate in Hz, computed from its EDID preferred timing (pixel
+    /// clock / total pixels per frame) while GOP was still alive — the one moment it is
+    /// readable (docs/gop.md). 0 = unknown (no EDID). The display service paces its
+    /// frame clock by it; without vblank this fixes the *rate*, never the *phase*.
+    refresh_hz: u32 = 0,
 
     /// Whether a usable framebuffer was handed over.
     pub fn present(self: Framebuffer) bool {