[iterate-3AA] Fix logo upside-down: no Y-flip on the clip-enabled NDC path

DEFECT 1 (logo upside down) ROOT + FIX. The publisher "SQUARE ENIX" logo
rendered vertically mirrored vs the canary oracle (white upright on black).

Measured (env-gated readback + texture-row + per-vertex dumps, all removed):
 - The K8888 logo texture decodes UPRIGHT (text in the top rows 1..161; the
   red dots sit at ~43% from the texture top). NOT a decoder row-order bug.
 - The logo geometry is a centered QuadList whose vertices are emitted in
   *clip space* (Y-UP, e.g. pos.y +0.085 top / -0.104 bottom), with the
   texture V mapped top->bottom (UV v 0.001 at the top vertex, 0.090 at the
   bottom). On both the Xbox 360 (D3D9) and wgpu, clip +Y maps to the
   framebuffer top — so a clip-space position is portable with NO Y-flip.
 - `compute_ndc_xy` unconditionally negated Y (the flip the *screen-space*
   pixel path legitimately needs). For the clip-enabled logo this swapped
   top<->bottom vertices while leaving the texture V unchanged, so the
   sampled sub-rect read bottom-up: red dots rendered at 58% from the top
   (a clean vertical mirror) instead of 43%.

FIX: keep the Y-flip only on the clip_disable (screen-space pixel) branch
where the framebuffer Y-down->wgpu Y-up flip is real; the clip-enabled
branch now passes clip-Y-up through identity. Readback after the fix: red
dots at 42% from the top (= texture's 43%) -> logo UPRIGHT, still centered.

DEFECT 2 (background) was already correct + faithful; 3Z's contradiction is
REFUTED by direct readback: the bg fill (vs 0x36660986 / ps 0xed732b5a,
fullscreen RectangleList) reads its real vertex color (raw 0x818000c7 =
-32896.5 as float) into r0, the PS exports it, and the GPUBUG-115 RB-UNORM
saturate (canary spirv_shader_translator.cc:3607) clamps it to 0 -> BLACK,
matching canary. The seed r0=(gvidx,...) does NOT show through (it's
overwritten by the color vfetch). No code change needed.

Readback of the full frame now matches canary: WHITE upright "SQUARE ENIX"
+ red dots on a BLACK field.

UI-capture-path only (`compute_ndc_xy` runs solely when frame_captures is
Some, i.e. --ui; None headless) -> deterministic core untouched, n50m
--gpu-inline --stable-digest exit 0 (DRAW_INDX 275 / K8888 decode 137,
identical across runs). cargo test --workspace green. Temp probes removed.

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

crates/xenia-gpu/src/draw_capture.rs

@@ -155,29 +155,40 @@ pub fn compute_ndc_xy(rf: &RegisterFile) -> ([f32; 2], [f32; 2]) {
 
     let (s, o);
     if clip_cntl & (1 << 16) != 0 {
-        // clip_disable: VS outputs render-target-pixel coords. Rescale the
-        // whole RT extent to [-1,1] (canary's huge-host-viewport path).
+        // clip_disable: VS outputs render-target-*pixel* coords (Y-DOWN: pixel
+        // y=0 is the top row of the render target). Rescale the whole RT extent
+        // to [-1,1] and FLIP Y so pixel-top → wgpu clip-top (canary's
+        // huge-host-viewport path; the framebuffer→clip flip is real here).
         let px2ndc_x = 2.0 / x_max;
         let px2ndc_y = 2.0 / y_max;
         let sx = scale_x * px2ndc_x;
         let ox = (off_x - x_max * 0.5 + add_x) * px2ndc_x;
         let sy = scale_y * px2ndc_y;
         let oy = (off_y - y_max * 0.5 + add_y) * px2ndc_y;
-        s = [sx, sy];
-        o = [ox, oy];
+        // Flip Y: pixel-Y-down → wgpu clip-Y-up.
+        s = [sx, -sy];
+        o = [ox, -oy];
     } else {
-        // Clipping enabled: the VS already emits clip space; the viewport
-        // scale/offset map clip→pixels. Convert to the host clip directly:
-        // host_ndc = guest_ndc (scale ~ 1) but still apply the abs-scale based
-        // remap canary uses. For the common enabled case the guest already
-        // outputs [-1,1] so scale=1/offset=0 except sign of Y. We approximate
-        // with identity XY + Y-flip (sufficient for non-screen-space draws;
-        // refined alongside depth in a follow-up).
+        // iterate-3AA (DEFECT 1 ROOT): clipping enabled → the VS already emits
+        // *clip-space* coordinates (Y-UP: +Y is the top of the screen), exactly
+        // the convention the Xbox 360's D3D9 and wgpu BOTH use for clip space
+        // (NDC +Y → framebuffer top in each API; the framebuffer Y-direction is
+        // an internal viewport detail handled identically by both). A clip-space
+        // position is therefore portable to wgpu with NO Y-flip. The previous
+        // code unconditionally negated Y (the same flip the screen-space pixel
+        // path needs), which mirrored the publisher logo vertically: its quad is
+        // centered (±0.085 around 0) so the *position* stayed centered, but the
+        // negation swapped top↔bottom vertices while the texture V was unchanged
+        // → the sampled sub-rect (UV v 0.001→0.090) read bottom-up → "SQUARE
+        // ENIX" rendered upside down in place. Measured (readback): the red dots
+        // sit at 43% from the texture top but rendered at 58% from the top
+        // (= a clean vertical mirror); removing the flip restores them to 43%.
+        // Identity XY (no flip) maps guest clip-Y-up straight to wgpu clip-Y-up.
         s = [1.0, 1.0];
         o = [0.0, 0.0];
+        return (s, o);
     }
-    // Flip Y for wgpu (render-target Y-down → clip Y-up).
-    ([s[0], -s[1]], [o[0], -o[1]])
+    (s, o)
 }
 
 /// Encode a [`PrimitiveType`] as the raw Xenos code used across the bridge.