[iterate-4A] diagnostics: XENIA_PROFILE wall-time profiler + probe/tooling snapshot

Handoff snapshot of the env-gated diagnostic scaffolding used across the
intro-video RE. Kept out of the milestone commits (645feb8..5573ac1) to keep
those clean; committed here so nothing is lost on handoff.

New — XENIA_PROFILE wall-time profiler (crates/xenia-gpu/src/prof.rs):
  Coarse buckets attributing playback wall time to interpreter (step_block),
  kernel HLE (call_export), block decode/cache (lookup_or_build), texture
  decode, host draw, and present; prints periodic snapshots (every 500M guest
  instr, or every 500 presents) + a clean-exit report. Hot path is gated on a
  cached is_on() (one relaxed load) so it is zero-cost when XENIA_PROFILE is
  unset. Call sites: main.rs run_superblock / parallel worker (step_block,
  lookup_or_build, call_export), texture_cache ensure_cached, render.rs present
  + dispatch_xenos_draws.

  First profile (movie playback, headless single-thread lockstep): effective
  ~35 MIPS; interpreter body ~40% @ ~95-102 MIPS; texture decode 0.3% (cache
  works); present ~0%; the rest is per-block dispatch + scheduler plumbing
  (~13 instr/block over 229M blocks). Overhead-bound, not interpreter-body
  bound; the levers are coarser execution units (superblock chaining) and
  ultimately a JIT.

Pre-existing read-only probe knobs (were uncommitted; env-gated, observe-only):
  XENIA_RET_CAPTURE_PC/_REG/_MEM, LOG_RESUMES, LOG_WAITS, LOG_SIGNAL,
  FORCE_TID, STARVE_LIMIT, INCUMBENT_PICK, INSTR_PER_MS, DUMP_FRAME,
  DUMP_WGSL, BIND_LOG, CONST_LOG, DISPATCH_REC, AUDIT_PC_TRACE.

Tooling: sylph-run.sh (movie oracle loop, 180s default timeout),
  zq.py (DuckDB disasm/xref helper).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xenia-gpu/src/texture_cache.rs

@@ -652,6 +652,7 @@ impl TextureCache {
             }
             self.restale_total += 1;
         }
+        let _prof_t0 = std::time::Instant::now();
         let bytes = match key.format {
             TextureFormat::K8 => decode_k8(&key, mem)?,
             TextureFormat::K8888 => decode_k8888_tiled(&key, mem)?,
@@ -659,9 +660,37 @@ impl TextureCache {
             TextureFormat::Dxt1 => decode_dxt1_tiled(&key, mem)?,
             TextureFormat::Dxt2_3 => decode_dxt23_tiled(&key, mem)?,
             TextureFormat::Dxt4_5 => decode_dxt45_tiled(&key, mem)?,
-            _ => return Err(DecodeError::UnsupportedFormat),
+            _ => {
+                // XENIA_TEX_REJECT_LOG diagnostic (STEP 87, read-only): the
+                // intro video uploads its YUV420 planes as `k_8` linear
+                // textures; if we have no decoder for the requested format we
+                // reject it here and it never reaches the GPU (→ black video).
+                // Rate-limited so a per-frame flood stays grep-able.
+                use std::sync::atomic::{AtomicUsize, Ordering};
+                static N: AtomicUsize = AtomicUsize::new(0);
+                let n = N.fetch_add(1, Ordering::Relaxed);
+                if n < 120 {
+                    tracing::warn!(
+                        fmt = ?key.format,
+                        w = key.width,
+                        h = key.height,
+                        base = format_args!("0x{:08x}", key.base_address),
+                        dim = ?key.dimension,
+                        pitch = key.pitch_texels,
+                        n,
+                        "TEX-REJECT: unsupported texture format (no host decoder)"
+                    );
+                }
+                return Err(DecodeError::UnsupportedFormat);
+            }
         };
         self.decodes_total += 1;
+        {
+            use crate::prof;
+            prof::add(&prof::TEXDEC_NS, _prof_t0.elapsed().as_nanos() as u64);
+            prof::add(&prof::TEXDEC_CALLS, 1);
+            prof::add(&prof::TEXDEC_BYTES, bytes.len() as u64);
+        }
         let entry = CachedTexture {
             key,
             version_when_uploaded: current_version,