test(check): ORACBUG-004 — sylpheed_n50m stable-digest oracle

Adds a regression-catcher golden for Sylpheed boot at -n 50M lockstep,
covering the first VdSwap pair (the n2m oracle is swap-blind because
the first VdSwap fires at ~18M instructions). The new --stable-digest
flag emits/compares only fields that are deterministic in lockstep:

  instructions, imports, unimpl, draws, swaps,
  unique_render_targets, shader_blobs_live, texture_cache_entries

Excluded:

  packets — empirically ±2-8% lockstep variance (GPU thread race per
    audit M11)
  resolves, interrupts_delivered, interrupts_dropped, texture_decodes —
    scheduling-sensitive under --parallel
  path — cwd-dependent

Empirical determinism: 3 consecutive lockstep -n 50M runs produce
byte-identical stable-digest output.

The n4b canonical-invocation golden the audit's recommended next sprint
also called for is deferred. Per audit memory `--parallel
--reservations-table` is pathologically slow (>32 min for -n 100M), so
-n 4B in that mode would be many hours per run, not the 5-15 min the
plan estimated. n4b will be captured one-shot post-renderer-unblock as
a manual artifact under audit-runs/post-fix/, not as a test golden. See
crates/xenia-app/tests/golden/README.md.

Test infrastructure:
- crates/xenia-app/tests/sylpheed_oracles.rs — invokes
  CARGO_BIN_EXE_xenia-rs against the ISO. Path resolved via SYLPHEED_ISO
  env var (skips gracefully if missing).
- #[ignore]-gated; run via:
    cargo test --release -p xenia-app --test sylpheed_oracles \\
      -- --ignored --nocapture

Closes ORACBUG-004 (P0). Partial: ORACBUG-006 (P1 deferred).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-app/tests/sylpheed_oracles.rs

@@ -0,0 +1,85 @@
+//! Sylpheed boot-sequence regression oracles.
+//!
+//! These goldens trigger `xenia-rs check` against the Project Sylpheed ISO and
+//! compare the resulting digest to a checked-in JSON file via `--stable-digest`,
+//! which excludes timing-sensitive counters (`packets`, `interrupts_*`,
+//! `resolves`, `texture_decodes`). The remaining fields are deterministic in
+//! lockstep at a fixed instruction budget — verified empirically across 3
+//! consecutive runs.
+//!
+//! Goldens are CIRCULAR per ORACBUG-001/002/003: they were captured by running
+//! the same code they validate. Treat them as **regression anchors** (catch
+//! drift from a known-good snapshot) not **correctness anchors** (no claim
+//! about absolute behavior). When a planned fix intentionally moves the
+//! digest (e.g. swap fix → `swaps` increments; renderer fix → `draws` becomes
+//! non-zero), re-baseline the golden as a separate commit.
+//!
+//! Tests are `#[ignore]`-gated because the runs take ~4 seconds each, which
+//! is unacceptable for the default `cargo test` cycle. Run explicitly:
+//!   cargo test --release -p xenia-app --test sylpheed_oracles -- --ignored --nocapture
+//!
+//! ISO path is read from the `SYLPHEED_ISO` env var, falling back to the
+//! repo-relative default. CI/contributors without the ISO will see the test
+//! skip gracefully.
+
+use std::process::Command;
+
+const ISO_DEFAULT: &str = "/home/fabi/RE Project Sylpheed/Project Sylpheed - Arc of Deception (USA, Europe) (En,Ja).iso";
+
+fn iso_path() -> String {
+    std::env::var("SYLPHEED_ISO").unwrap_or_else(|_| ISO_DEFAULT.to_string())
+}
+
+fn run_oracle(label: &str, max_instr: u64, golden_rel: &str) {
+    let bin = env!("CARGO_BIN_EXE_xenia-rs");
+    let iso = iso_path();
+    if !std::path::Path::new(&iso).exists() {
+        eprintln!("{label}: iso not found at {iso}; set SYLPHEED_ISO to override. SKIPPING.");
+        return;
+    }
+
+    // Resolve the golden path relative to the test's CARGO_MANIFEST_DIR so the
+    // test runs correctly from any cwd.
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let golden = std::path::Path::new(manifest_dir).join(golden_rel);
+    assert!(
+        golden.exists(),
+        "{label}: golden file missing at {}",
+        golden.display()
+    );
+
+    let max_instr_str = max_instr.to_string();
+    let golden_str = golden.to_string_lossy().to_string();
+
+    let out = Command::new(bin)
+        .args([
+            "check",
+            &iso,
+            "-n",
+            &max_instr_str,
+            "--stable-digest",
+            "--expect",
+            &golden_str,
+        ])
+        .output()
+        .expect("failed to spawn xenia-rs");
+
+    if !out.status.success() {
+        eprintln!(
+            "{label}: STDOUT:\n{}\nSTDERR:\n{}",
+            String::from_utf8_lossy(&out.stdout),
+            String::from_utf8_lossy(&out.stderr),
+        );
+        panic!("{label}: digest mismatch (exit {:?})", out.status.code());
+    }
+}
+
+/// Sylpheed boot to first VdSwap pair, captured at -n 50M lockstep.
+/// Catches regressions in: addi/addic semantics, kernel HLE for VdSwap path,
+/// thread spawning, file I/O for sound/config. With Phase A's swap fix landed,
+/// `swaps` should be 2 and `draws` 0 (Phase E gates draws>0).
+#[test]
+#[ignore = "long-running; run via `cargo test ... -- --ignored sylpheed_n50m`"]
+fn sylpheed_n50m() {
+    run_oracle("sylpheed_n50m", 50_000_000, "tests/golden/sylpheed_n50m.json");
+}