[iterate-3X] Real splash logo geometry renders: fix vertex-fetch const_index_sel + per-draw submit

Two readback-proven root-cause fixes make the publisher-logo QuadList draw
land its REAL captured vertex buffer (the texture was already correct from
3V). REFUTES iterate-3W's "logo geometry is auto-generated from vertex_id":
the logo IS sourced from a 4-vertex QuadList buffer at guest physical
0x0adf60f0 (measured), it was just resolved at the wrong fetch-constant
register.

GPUBUG-110 (vertex fetch const_index_sel dropped). The Xenos vertex-fetch
instruction encodes const_index (w0[20:24]) AND const_index_sel (w0[25:26]);
the full constant index is const_index*3 + const_index_sel (canary
ucode.h:700), packed 3 two-dword constants per 6-dword register group.
ucode/fetch.rs decoded only const_index and read sub-slot 0 (fc*6). The logo
vfetch is const_index=31, sel=2 -> the real base lives at reg 0x48BE, but
ours read 0x48BA which held an unused 0x00000001 (base=0,size=0) slot. So
resolve_vertex_window returned None -> has_real_vertices=false -> the logo
fell to the procedural fullscreen magenta fallback. Fix: decode
const_index_sel, add VertexFetch::const_reg_offset() = const_index*6 + sel*2,
and use it in both draw_capture.rs (capture) and translator.rs (the WGSL
endian term + no-window fallback base; the old expression there read the
src_reg bits, not the const index). Measured: logo now resolves a 24-dword
(4 verts x stride 6) window, base 0x0adf60f0.

GPUBUG-111 (single batch encoder = last-draw-wins vertex data). In wgpu every
queue.write_buffer staged before a single queue.submit is applied before ANY
command in that submit runs. dispatch_xenos_captures recorded the whole batch
into one encoder + one submit, so every draw read only the LAST draw's vertex
buffer / per-draw uniforms. The logo quad therefore sampled the trailing
fullscreen background quad's vertices and rasterized nothing where the logo
was. Fix: submit one encoder per draw (frontbuffer LoadOp::Load composites
identically). Measured (env-gated readback, removed): with this fix the logo
draw in isolation renders real varied texels (e.g. (225,17,22)/(255,255,0))
in a centered strip (~20k px), vs 100% navy before.

Determinism: all changes are UI-side (xenia-ui replay) or the UI translator /
capture path (frame_captures None in headless); the fetch.rs field addition
is purely additive and does not change any existing decoded value. Verified
the deterministic core unchanged: check -n50M --gpu-inline --stable-digest
exit 0 and all 136 metric counters byte-identical across two runs. All temp
probes removed. cargo test --workspace green; new regression test
vertex_fetch_const_index_sel_and_reg_offset.

Known remaining (next iterate): a fullscreen flat QuadList (ps 0x03b79081,
vertex color green, no texture) and other textureless draws overpaint the
logo in the full composite (their per-draw blend/alpha render state is not
yet replayed, and draw order alternates bg/logo). The logo artwork renders
correctly in isolation; the composite is not yet clean.

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

crates/xenia-ui/src/render.rs

@@ -742,11 +742,16 @@ impl RenderState {
             return 0;
         }
         let mut real_count = 0u32;
-        let mut encoder = self
-            .device
-            .create_command_encoder(&wgpu::CommandEncoderDescriptor {
-                label: Some("xenos capture replay"),
-            });
+        // iterate-3X (GPUBUG-111): each captured draw uploads its OWN vertex
+        // window + per-draw constants + shader via `queue.write_buffer`. In
+        // wgpu all `write_buffer` calls staged before a single `queue.submit`
+        // are applied *before any* command in that submit executes — so a single
+        // encoder for the whole batch made every draw read only the LAST draw's
+        // vertex buffer / uniforms (the splash logo quad sampled the fullscreen
+        // background quad's vertices → nothing rendered where the logo was).
+        // Submit ONE encoder PER draw so each draw's writes land before its own
+        // pass. The frontbuffer uses `LoadOp::Load`, so per-draw submits still
+        // composite over each other exactly like before.
         for cap in captures {
             // iterate-3T: bind this draw's REAL decoded texture (keyed off the
             // active PS's tfetch slot, attached in `gpu_system`) so the textured
@@ -831,6 +836,11 @@ impl RenderState {
                 ndc_scale: if cap.has_real_vertices { cap.ndc_scale } else { [0.0, 0.0] },
                 ndc_offset: if cap.has_real_vertices { cap.ndc_offset } else { [0.0, 0.0] },
             };
+            let mut encoder = self
+                .device
+                .create_command_encoder(&wgpu::CommandEncoderDescriptor {
+                    label: Some("xenos capture replay (per-draw)"),
+                });
             if use_translated
                 && let Some(p) = self.xenos_pipeline.translated_pipeline(cap.vs_key, cap.ps_key)
             {
@@ -853,8 +863,8 @@ impl RenderState {
                 self.xenos_dispatches_interpreter =
                     self.xenos_dispatches_interpreter.saturating_add(1);
             }
+            self.queue.submit(std::iter::once(encoder.finish()));
         }
-        self.queue.submit(std::iter::once(encoder.finish()));
         self.xenos_draws_rendered = self
             .xenos_draws_rendered
             .saturating_add(captures.len() as u64);