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/kernel/kernel.zig

@@ -200,8 +200,8 @@ fn kmain(boot_information: *const BootInformation) noreturn {
         // Publish the loader's framebuffer as a claimable `display` device, so a
         // user-space display service can take it over the same claim + mmio_map path as
         // any other hardware (it is not firmware-discovered; it rides the boot handoff).
-        if (devices_broker.seedDisplay(fb.base, fb.width, fb.height, fb.pitch, @intFromEnum(fb.format))) |display_id| {
-            log.print("/system/kernel: framebuffer device {d} seeded ({d}x{d}, pitch {d}, write-combining)\n", .{ display_id, fb.width, fb.height, fb.pitch });
+        if (devices_broker.seedDisplay(fb.base, fb.width, fb.height, fb.pitch, @intFromEnum(fb.format), fb.refresh_hz)) |display_id| {
+            log.print("/system/kernel: framebuffer device {d} seeded ({d}x{d}, pitch {d}, {d} Hz, write-combining)\n", .{ display_id, fb.width, fb.height, fb.pitch, fb.refresh_hz });
         }
 
         // Install the device-IRQ trampolines, so a driver's irq_bind has vectors to