[iterate-3AJ] Present-anchor vsync so the splash logo fade-in renders

The publisher/dev splash logo's intro fade-IN was skipped: the logo
popped in at full brightness instead of ramping dim->bright like the
canary oracle. Root (measured, iterate-3AF/3AI): ours' guest vsync
counter is fed by a fixed-instruction-quantum proxy (one vsync per
150k retired instructions). During the ~1.1s splash asset-load the
title's frame pump runs ~10M instructions inside a single guest frame,
so the proxy fired ~66 vsyncs in that one frame. The pump's per-frame
delta (counter_now - counter_last) was therefore ~66 on the first tick,
which the anim tick (sub_823CDBF8) divides into the fade counter
[item+72] @ 0x40c0add0 -> the counter JUMPED 0->0x42(66) in one step,
landing past the fade-in region. Canary's wall-clock 60Hz vblank
advances ~1 per heavy load frame, so its counter ramps smoothly 0->66
and the fade-in renders.

Fix: anchor the lockstep vsync ticker to the guest's real present rate
(VdSwap count), mirroring real hardware where the title double-buffers
at vblank, so one heavy guest frame advances the vsync counter by ~1
instead of ~66.

- interrupts.rs: tick_vsync_instr now takes the live present count.
  Two regimes: (1) bootstrap, before the guest's first present, keeps
  the original fixed instruction quantum unchanged -- the iterate-2W
  present-loop bootstrap needs vsyncs delivered BEFORE it can present
  (measured: callback registered ~6M instr, first delivered vsync and
  first present coincide; pure present-driven vsync would deadlock).
  (2) present-anchored, after the first present: one vblank per present,
  plus a small DRY_FALLBACK_CAP=4 instruction-quantum fallback per dry
  window so a non-presenting frame still ticks a few vsyncs (a small
  ramp like canary's 0/5/10/2/1...) without re-spiking to 66.
- handle.rs: cheap GpuBackend::swaps_seen() accessor.
- main.rs: pass the live present count into the lockstep ticker.

Not masking: the fade dt/counter is never clamped or synthesized; the
guest naturally computes a smooth dt once vblank tracks presents.

Verified:
- V1: fade counter 0x40c0add0 now ramps 0,6,8,10,12,13,+1... (was a
  0->0x42 jump; direct baseline-vs-fix mem-watch).
- V2 (--ui readback via per-frame logo vertex-alpha): logo alpha ramps
  102,136,204,221,238,254 (dim->bright fade-IN) vs baseline all 255
  (pop-in). Real artwork (has_real_vertices) still renders; milestone-1
  intact.
- V3: 150M boot progression intact -- texture_decodes=2, RTs=2,
  tex_cache=1 unchanged; draws/swaps higher (tighter present loop),
  1B sanity linear, no stall/collapse.
- V4: 50M --gpu-inline --stable-digest byte-identical 2x; golden
  re-baselined intentionally (pacing-only delta: draws 718->1274,
  swaps 147->259; structural fields unchanged). 688 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/xenia-gpu/src/handle.rs

@@ -444,6 +444,23 @@ impl GpuBackend {
         }
     }
 
+    /// Current guest present (`VdSwap`) count. Cheap single-field read used
+    /// by the present-anchored vsync ticker (iterate-3AJ) every scheduler
+    /// round. Inline mode reads the live counter directly; threaded mode
+    /// reads the last-published digest mirror under a brief lock (the
+    /// `--parallel` path uses the wall-clock vsync ticker anyway, so the
+    /// exact freshness here is not load-bearing).
+    pub fn swaps_seen(&self) -> u64 {
+        match self {
+            GpuBackend::Inline(s) => s.stats.swaps_seen,
+            GpuBackend::Threaded(h) => h
+                .digest
+                .lock()
+                .map(|d| d.stats.swaps_seen)
+                .unwrap_or(0),
+        }
+    }
+
     /// Forward [`GpuSystem::has_pending_interrupts`] under inline mode;
     /// under threaded mode peek the `int_rx` channel.
     pub fn has_pending_interrupts(&self) -> bool {