[iterate-2X] Texture pipeline: un-stub RectangleList + draw-time texture decode

Two faithful, deterministic GPU-backend changes that make the texture path
correct for whatever textured draw the splash eventually dispatches. Both are
currently inert on Sylpheed (the textured logo draw is still gated downstream
— see below), but neither shifts the stable-digest golden, so they land safely.

1. Un-stub RectangleList primitive expansion (primitive.rs). The splash submits
   2819 RectangleList draws at 200M, all of which were REJECTED by the P3 stub
   (`gpu.primitive.rejected{rectangle_list}`) → only ~592 flat point/quad draws
   rasterized. Mirror canary's intent (primitive_processor.cc:389-456
   kRectangleListAsTriangleStrip) within our CPU index-rewrite idiom: emit each
   rect's 3 real vertices as one TriangleList triangle (v0,v1,v2), rejected=false,
   faithful host_vertex_count. The full quad (synthesized 4th corner v3=v0+v2-v1)
   needs real vertex fetch in vs_main — left as a documented TODO. Rejection
   warnings drop 2819→0.

2. Draw-time texture decode keyed off the active PS's real tfetch slots
   (gpu_system.rs + exports.rs vd_swap). Previously vd_swap decoded a hardcoded
   fetch-constant slot 0 at swap time. Now the DRAW handler parses the bound
   pixel shader (ucode::parse_shader), collects its tfetch fetch_const slots via
   new shader_metrics::tfetch_slots, reads each 6-dword fetch constant, and
   decode+caches it into GpuSystem::last_draw_textures. vd_swap publishes the
   first of these (UI binds one texture today), falling back to the legacy slot-0
   probe on flat-only frames. New span_max_version helper walks page_version over
   the trait (draw-time &dyn MemoryAccess lacks the heap's inherent
   max_page_version). Pure function of guest writes — deterministic.

Status: texture_decodes stays 0 on Sylpheed because all 6 live shaders are flat
(no tfetch); canary's textured logo shaders E59B2B3D/F7B1457 are not yet
dispatched by ours (a downstream title-state gate, the next frontier). The full
P5 decode→publish→upload→sample path is already wired; this makes the decode
side key off the real shader instead of a guess.

Validation: stable-digest golden sylpheed_n50m unchanged (draws=718 swaps=147
tex=0), regenerated twice byte-identical; 200M run shows 0 RectangleList
rejections. cargo test --workspace green (677, +2: rectangle_list_expansion,
tfetch_slots_extracts_texture_fetch_constants). No temp hooks. Branch only;
not pushed/merged.

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

crates/xenia-gpu/src/gpu_system.rs

@@ -78,6 +78,30 @@ pub fn physical_to_backing(addr: u32) -> u32 {
     }
 }
 
+/// Max guest page-version over the `[base, base+len)` span, walking 4 KiB
+/// pages via the `MemoryAccess` trait's `page_version`.
+///
+/// The concrete heap exposes an inherent `max_page_version(base, len)`, but
+/// the draw handler only holds `&dyn MemoryAccess` (which carries the coarser
+/// `page_version(addr)` accessor). This is byte-equivalent to
+/// `heap::max_page_version` and stays a pure function of the per-page write
+/// counters (no wall-clock), so texture-decode timing remains deterministic.
+fn span_max_version(mem: &dyn MemoryAccess, base: u32, len: u32) -> u64 {
+    const PAGE: u32 = 0x1000;
+    let last = base.saturating_add(len.saturating_sub(1));
+    let mut page = base & !(PAGE - 1);
+    let last_page = last & !(PAGE - 1);
+    let mut max = 0u64;
+    loop {
+        max = max.max(mem.page_version(page));
+        if page >= last_page {
+            break;
+        }
+        page = page.wrapping_add(PAGE);
+    }
+    max
+}
+
 /// Cached Xenos microcode blob, produced by `PM4_IM_LOAD*` packets.
 #[derive(Debug, Clone)]
 pub struct ShaderBlob {
@@ -400,6 +424,12 @@ pub struct GpuSystem {
     /// on every texture-fetch resolution; the UI thread sees the decoded
     /// bytes via `UiBridge::publish_texture`.
     pub texture_cache: crate::texture_cache::TextureCache,
+    /// P5b: textures decoded at the most recent `PM4_DRAW_INDX*`, keyed off
+    /// the *active* pixel shader's real `tfetch` fetch-constant slots (not a
+    /// hardcoded slot). `vd_swap` publishes the first of these to the UI so
+    /// the replay binds the texture the draw actually samples. Cleared and
+    /// repopulated each draw; empty when the active PS issues no `tfetch`.
+    pub last_draw_textures: Vec<(crate::texture_cache::TextureKey, Vec<u8>)>,
     /// 10 MiB shadow of the Xenos EDRAM. Written by clear-resolves and
     /// (future) host-render-target readback; read by the resolve byte-copy
     /// path that writes tiled pixels into guest memory. Allocated once at
@@ -431,6 +461,7 @@ impl GpuSystem {
             rt_cache: crate::render_target_cache::RenderTargetCache::new(),
             last_resolve: None,
             texture_cache: crate::texture_cache::TextureCache::new(),
+            last_draw_textures: Vec::new(),
             edram: crate::edram::ShadowEdram::new(),
         }
     }
@@ -1265,6 +1296,60 @@ impl GpuSystem {
                 );
                 self.last_draw = Some(ds);
                 self.last_primitive = Some(processed);
+
+                // P5b: decode the textures the *active pixel shader* actually
+                // samples. Parse the bound PS, collect its `tfetch`
+                // fetch-constant slots, read each 6-dword fetch constant from
+                // the register file, and decode+cache it. `vd_swap` publishes
+                // the result. Empty for flat (no-tfetch) shaders — the
+                // dominant case on Sylpheed's current splash, where this stays
+                // inert until the textured logo draw is reached.
+                self.last_draw_textures.clear();
+                if let Some(ps_key) = self.active_ps_key {
+                    // Collect slots under an immutable borrow of `shader_blobs`,
+                    // then drop it before mutating `texture_cache`.
+                    let slots: Vec<u8> = match self.shader_blobs.get(&ps_key) {
+                        Some(blob) => {
+                            let parsed = crate::ucode::parse_shader(&blob.dwords);
+                            crate::shader_metrics::tfetch_slots(&parsed)
+                        }
+                        None => Vec::new(),
+                    };
+                    for slot in slots {
+                        let mut fetch6 = [0u32; 6];
+                        for (k, w) in fetch6.iter_mut().enumerate() {
+                            *w = self
+                                .register_file
+                                .read(CONST_BASE_FETCH + slot as u32 * 6 + k as u32);
+                        }
+                        let Some(key) = crate::texture_cache::decode_fetch_constant(fetch6) else {
+                            continue;
+                        };
+                        let bi = key.format.block_info();
+                        let span_bytes = (key.pitch_texels as u32)
+                            * (key.height as u32)
+                            * (bi.bytes_per_block as u32)
+                            / (bi.block_w as u32);
+                        let version = span_max_version(mem, key.base_address, span_bytes.max(4));
+                        match self.texture_cache.ensure_cached(key, version, mem) {
+                            Ok(entry) => {
+                                self.last_draw_textures.push((entry.key, entry.bytes.clone()));
+                                metrics::counter!(
+                                    "gpu.texture.decode",
+                                    "fmt" => format!("{:?}", key.format),
+                                )
+                                .increment(1);
+                            }
+                            Err(e) => {
+                                metrics::counter!(
+                                    "gpu.texture.reject",
+                                    "reason" => format!("{e:?}"),
+                                )
+                                .increment(1);
+                            }
+                        }
+                    }
+                }
             }
             pm4::PM4_SET_CONSTANT | pm4::PM4_SET_SHADER_CONSTANTS => {
                 // payload[0] = offset_type — bits[10:0] index, bits[23:16] type