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 },

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,

system/services/display/display.zig

@@ -287,7 +287,8 @@ fn attachScanout(stride: u32, width: u32, height: u32, format: u32, capability:
 /// After the native upgrade is verified, prove the runtime-resolution-change and fenced-present
 /// paths: query the driver's modes, switch to one that differs from the current, re-composite
 /// the whole screen at the new size, and confirm the backend now reports that geometry. The
-/// present goes through the driver's fenced flush, so a clean present is a vsync present.
+/// present goes through the driver's fenced flush, so a clean present is a *fenced* present —
+/// completion-acknowledged and tear-free, not vblank-paced (docs/display-v2.md).
 fn modesetSelfCheck() void {
     if (!backend.canModeSet()) return;
     var mode_list: [4]backend_mod.Mode = undefined;
@@ -319,7 +320,7 @@ fn modesetSelfCheck() void {
     if (now.width == wanted.width and now.height == wanted.height) {
         var line: [80]u8 = undefined;
         _ = system.write(std.fmt.bufPrint(&line, "display: mode set to {d}x{d}, verified\n", .{ now.width, now.height }) catch "display: mode set, verified\n");
-        if (backend.hasVsync()) _ = system.write("display: vsync present ok\n");
+        if (backend.hasFencedPresent()) _ = system.write("display: fenced present ok\n");
     } else {
         _ = system.write("display: mode set FAILED (geometry unchanged)\n");
     }