[iterate-3AE] Fix spurious WHITE TRIANGLE flashing before each splash logo

The publisher and developer splash logos rendered correctly, but a
fullscreen OPAQUE WHITE diagonal half-triangle flashed at boot, before
each logo, and persisted across the dev-logo transition — canary shows a
black background there. Readback-isolated it (env-gated frontbuffer grid
+ per-draw inventory, both removed) to the background-fill draws.

ROOT (measured, refutes the prior "saturate/interpreter/depth" guesses):
the position-only VS `0xd4c14f46` (one vfetch → oPos; exports NO color)
paired with PS `0xed732b5a` (`ocolor0 = interp0`). The iterate-3T
translator seeded `ointerp[0] = (1,1,1,1)` "so a VS that only exports
position still yields a visible non-zero color" — a debug FAKE: it
injects white that no guest value backs. So that fill's interp0 stayed
white → opaque-white fullscreen triangle. Vertex windows of a WHITE frame
and a steady BLACK frame were byte-identical; served_translated=true for
all of them and depth is disabled in the replay, so the white came purely
from the injected seed, not saturate/interp/depth.

FIX (UI-translator only, golden byte-identical):
- translator.rs: default un-exported interpolators to (0,0,0,0) instead
  of seeding interp0 white. A position-only VS now contributes nothing
  visible under its real blend (RGB=0 → black; A=0 → premult transparent),
  matching canary; every VS that really exports interp0 (the logo
  `0x03b7b020`, the color fill `0x36660986`) overwrites the seed → logos
  unaffected.
- app.rs: clear the splash frontbuffer to BLACK, not the iterate-3S navy
  placeholder `[0.04,0.04,0.06]` (never matched to the guest). The fill is
  a fullscreen Xbox-360 RectangleList drawn as a single triangle in the
  replay (4th implied corner not yet synthesized), so its uncovered half
  exposed the clear; black makes the transition uniformly black like the
  oracle. (Full RectangleList→rectangle expansion is a separate follow-up.)

READBACK (env-gated, removed): white-heavy frames 200+ → 0; navy frames
240 → 0; transition frames uniformly black; the publisher logo (white
text + red dots) and the developer logos (colored, on black) still
render. Determinism: changes feed only the UI translator/clear; n50m
--gpu-inline --stable-digest byte-identical 2× and matches the committed
golden (--expect exit 0). cargo test --workspace 686 passed.

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

crates/xenia-ui/src/app.rs

@@ -369,7 +369,25 @@ impl ApplicationHandler<SwapEvent> for App {
                             .map(|s| s.frame_index)
                             .unwrap_or(0);
                         if frame_idx != self.last_xenos_swap_frame {
-                            rs.clear_frontbuffer([0.04, 0.04, 0.06, 1.0]);
+                            // iterate-3AE: clear to BLACK, matching canary's
+                            // splash background. The old navy `[0.04,0.04,0.06]`
+                            // was an iterate-3S debug placeholder never matched
+                            // to the guest. The splash background-fill draw is a
+                            // full-screen Xbox-360 RectangleList (3 verts → a HW
+                            // rectangle covering the whole screen); the UI replay
+                            // draws it as a single triangle (the 4th implied
+                            // corner isn't synthesized), so only the diagonal
+                            // half is covered. With a navy clear the uncovered
+                            // half showed a navy diagonal in the brief
+                            // pre/inter-logo transition frames (where that fill
+                            // is the only coverage). Canary's background there is
+                            // black, and the guest's fill itself resolves to
+                            // black, so a black clear makes the uncovered half
+                            // match — the transition is uniformly black like the
+                            // oracle. (Full RectangleList→rectangle expansion is
+                            // the deeper fix and a separate follow-up; under a
+                            // black clear the half-coverage is invisible.)
+                            rs.clear_frontbuffer([0.0, 0.0, 0.0, 1.0]);
                             self.last_xenos_swap_frame = frame_idx;
                         }
                         let delta = (draws_total - already) as u32;