display: stop calling the fenced present 'vsync' — it isn't

The virtio-gpu present fence completes when the device has consumed the
frame: real completion feedback, and tear-freedom by snapshot semantics.
It is not a vblank — base virtio-gpu 2D has no display-refresh event at
all (Linux fakes one with a timer), so nothing paces presents to the
monitor. The code and docs claimed vsync anyway; now they don't.

- backend.hasVsync -> hasFencedPresent, with an honest doc comment
- marker 'display: vsync present ok' -> 'display: fenced present ok'
  (display-modeset test expectation updated, passes)
- display-v2.md gains a 'Fenced is not vsync' note; the vsync claims in
  both v2 docs are corrected
- true vsync arrives with a native driver's vblank IRQ, or approximated
  by a compositor frame clock

build.zig

@@ -854,7 +854,7 @@ pub fn build(b: *std.Build) void {
     // The VGA device still supplies the boot (GOP) framebuffer the compositor starts
     // on; the virtio-gpu function is discovered by the device-manager stack, its
     // driver announces a shared scanout, and the compositor upgrades off the GOP
-    // floor to fenced — vsync, tear-free — native presents (docs/display-v2.md).
+    // floor to fenced, tear-free native presents (docs/display-v2.md).
     // This is the interactive twin of the `display-native` test case, and 512M
     // matches it (the whole driver stack + the compositor's surfaces at once).
     // QEMU shows one head per adapter: pick the virtio-gpu head in the View menu