[iterate-3O] Real-render slice: replay guest geometry in --ui (Route A)

Replace the synthetic placeholder triangle in the --ui window with the
splash's REAL guest geometry, proving the faithful-render pipe end to end.

Architecture: Route A (UI-side replay). A per-draw capture channel carries
each PM4_DRAW_INDX*'s real state to the UI, which replays it through the
existing wgpu Xenos pipeline. The deterministic headless core is untouched:
capture is gated on an Option<Vec<DrawCapture>> that is None in headless
mode and only enabled on the --ui path, so the --gpu-inline n50m golden is
byte-identical (verified 2x).

The hard part was sourcing real vertices. The WGSL VS already does
format-aware vertex fetch from the b4 storage buffer at the address from the
fetch constant -- but b4 was never populated and the fetch address is an
absolute guest dword address. The slice:
  * xenia-gpu/draw_capture.rs: parse the active VS, find its first vertex
    fetch, read that fetch constant, copy a bounded window of guest memory
    at the fetch base. Best-effort: has_real_vertices=false falls back to
    procedural geometry (never fabricated pixels).
  * gpu_system.rs: accumulate one DrawCapture per draw into frame_captures.
  * exports.rs (vd_swap): drain + publish the frame's captures to the UI.
  * ui_bridge/bridge.rs: new publish_geometry channel + UiHandles.geometry.
  * WGSL (interp + translator): rebase the absolute fetch address by a new
    DrawConstants.vertex_base_dwords so it indexes the uploaded window.
  * render.rs: dispatch_xenos_captures uploads each draw's real vertex
    window + matching shader, issues real DrawRequests (real prim type,
    host vertex count, vs/ps keys).
  * app.rs: prefer the real-capture replay; HUD adds real-geo=N counter.

Verified in --ui on Sylpheed: "first Xenos capture batch replayed (real
geometry) captures=24 real_vertex_draws=24" -- all draws resolved a real
guest vertex window; WGSL compiles; no validation errors over 1616 swaps.

Still synthetic-free but not yet pixel-perfect: textures/UVs, DMA index
buffers (auto-index only for now), and kCopy resolve routing are staged
for follow-ups. Faithful: real vertex data, prim types, shaders, constants.

cargo test --workspace green; n50m golden unchanged (2x byte-identical).

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

crates/xenia-gpu/src/translator.rs

@@ -94,7 +94,7 @@ struct XenosDrawConstants {
     draw_index: u32,
     vertex_count: u32,
     prim_kind: u32,
-    _pad: u32,
+    vertex_base_dwords: u32,
 };
 
 struct XenosConstants {
@@ -418,7 +418,9 @@ impl EmitCtx {
             "{{ let fc0 = xenos_consts.fetch[{fc0_idx}u]; \
              let fc1 = xenos_consts.fetch[{fc1_idx}u]; \
              let endian = fc1 & 0x3u; \
-             let base = (fc0 & 0xFFFFFFFCu) >> 2u; \
+             let abs_base = (fc0 & 0xFFFFFFFCu) >> 2u; \
+             let base = select(abs_base, abs_base - draw_ctx.vertex_base_dwords, \
+                 draw_ctx.vertex_base_dwords != 0u && abs_base >= draw_ctx.vertex_base_dwords); \
              let vidx = u32(r[{src_reg}u].x); \
              let addr = base + vidx * 4u; \
              let n = arrayLength(&vertex_buffer); \