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

system/services/display/backend.zig

@@ -26,7 +26,7 @@ var device_table: [64]device.DeviceDescriptor = undefined;
 /// framebuffer write-combining as the front buffer, and keeps a cacheable back buffer of
 /// the same geometry as the compose target. `present` streams the damaged rectangle from
 /// the back buffer to the LFB (sequential WC writes; the LFB is never read). No mode-set,
-/// no vsync — the portable floor (docs/display-v2.md).
+/// no present fence — the portable floor (docs/display-v2.md).
 pub const Gop = struct {
     device_id: u64,
     front: [*]volatile u8, // the LFB (write-combining)
@@ -263,9 +263,13 @@ pub const Backend = union(enum) {
             .virtio => true,
         };
     }
-    /// Whether this backend has a vblank/fence for tear-free present (virtio-gpu: yes, V5 — every
-    /// flush is fenced, so the device signals completion when the frame is actually on screen).
-    pub fn hasVsync(self: *const Backend) bool {
+    /// Whether this backend's present is **fenced** — it completes only once the device has
+    /// consumed the frame (virtio-gpu: every flush carries a fence the used-ring ack waits on).
+    /// A fence gives completion feedback and tear-free snapshot presents; it is *not* vblank —
+    /// nothing paces presents to the display's refresh (base virtio-gpu 2D has no vblank event
+    /// at all). True vsync needs a native driver's vblank interrupt. See docs/display-v2.md,
+    /// "Fenced is not vsync".
+    pub fn hasFencedPresent(self: *const Backend) bool {
         return switch (self.*) {
             .gop => false,
             .virtio => true,