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/drivers/virtio-gpu/virtio-gpu.zig

@@ -53,8 +53,9 @@ const offered_modes = [_]Mode{ .{ .width = 640, .height = 480 }, .{ .width = 800
 var current_width: u32 = offered_modes[0].width;
 var current_height: u32 = offered_modes[0].height;
 
-/// Monotonic fence id for fenced (vsync) flushes; the device signals the fence when the flush
-/// is complete, which its used-ring ack already gates our synchronous present on.
+/// Monotonic fence id for fenced flushes; the device signals the fence when the flush is
+/// complete, which its used-ring ack already gates our synchronous present on. Completion
+/// feedback, not vblank — nothing here is paced to the display's refresh.
 var fence_next: u64 = 1;
 
 /// Whether the device offered VIRTIO_GPU_F_EDID, so `get_edid` is worth issuing.
@@ -492,8 +493,9 @@ fn presentFull() bool {
         if (command_nodata(@sizeOf(vg.TransferToHost2d)) != ok_nodata) return false;
     }
     {
-        // A fenced flush (vsync): the device signals the fence when the frame is actually on
-        // screen — which its used-ring ack, what our synchronous submit waits on, already gates.
+        // A fenced flush: the device signals the fence once it has consumed the frame — which
+        // its used-ring ack, what our synchronous submit waits on, already gates. Completion
+        // feedback and a tear-free snapshot, not vblank pacing.
         const request = requestAt(vg.ResourceFlush);
         request.* = .{
             .hdr = .{ .type = @intFromEnum(vg.CmdType.resource_flush), .flags = vg.flag_fence, .fence_id = fence_next },