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/src/main.rs

@@ -226,6 +226,13 @@ enum Commands {
         /// Optional golden digest JSON; `check` exits non-zero on mismatch
         #[arg(long)]
         expect: Option<String>,
+        /// Emit/compare only stable fields (excludes timing-sensitive counters
+        /// like `packets`, `interrupts_delivered`, `resolves`). Required for any
+        /// golden captured under `--parallel`; recommended for lockstep goldens
+        /// at -n ≥50M because `packets` has empirical ±2–8% jitter from a GPU
+        /// thread race.
+        #[arg(long)]
+        stable_digest: bool,
         /// Force the threaded GPU backend (default at M1.9).
         #[arg(long)]
         gpu_thread: bool,
@@ -324,6 +331,7 @@ fn main() -> Result<()> {
             max_instructions,
             out,
             expect,
+            stable_digest,
             gpu_thread,
             gpu_inline,
             reservations_table,
@@ -333,6 +341,7 @@ fn main() -> Result<()> {
             max_instructions,
             out.as_deref(),
             expect.as_deref(),
+            stable_digest,
             gpu_thread,
             gpu_inline,
             reservations_table,
@@ -528,6 +537,7 @@ fn cmd_exec(
         parallel,
         None,
         None,
+        false,
     )
 }
 
@@ -538,6 +548,7 @@ fn cmd_check(
     max_instructions: u64,
     out: Option<&str>,
     expect: Option<&str>,
+    stable_digest: bool,
     gpu_thread: bool,
     gpu_inline: bool,
     reservations_table: bool,
@@ -561,6 +572,7 @@ fn cmd_check(
         parallel,
         out,
         expect,
+        stable_digest,
     )
 }
 
@@ -582,6 +594,7 @@ fn cmd_exec_inner(
     parallel: bool,
     digest_out: Option<&str>,
     digest_expect: Option<&str>,
+    stable_digest: bool,
 ) -> Result<()> {
     let started = Instant::now();
     let data = load_xex_data(path)?;
@@ -1190,10 +1203,14 @@ fn cmd_exec_inner(
             // catch any drift between runs.
             if digest_out.is_some() || digest_expect.is_some() {
                 let digest = RunDigest::capture(path, &kernel, &stats);
-                let json = digest.to_json();
+                let json = if stable_digest {
+                    digest.stable_fields_json()
+                } else {
+                    digest.to_json()
+                };
                 if let Some(out_path) = digest_out {
                     std::fs::write(out_path, &json)?;
-                    info!(out = out_path, "run digest written");
+                    info!(out = out_path, stable = stable_digest, "run digest written");
                 } else {
                     println!("{json}");
                 }
@@ -1340,6 +1357,34 @@ impl RunDigest {
             self.texture_decodes,
         )
     }
+
+    /// Stable-fields-only digest for goldens that need to survive non-determinism.
+    /// Excludes timing-sensitive counters: `packets` has documented ±2.5–8% lockstep
+    /// noise from a GPU thread race; `resolves`, `interrupts_delivered`,
+    /// `interrupts_dropped`, and `texture_decodes` are scheduling-sensitive under
+    /// `--parallel`. Also omits `path` (cwd-dependent). The remaining fields are
+    /// deterministic in lockstep at a fixed instruction budget. Use via
+    /// `--stable-digest`.
+    fn stable_fields_json(&self) -> String {
+        format!(
+            "{{\n  \"instructions\": {},\n  \
+             \"imports\": {},\n  \
+             \"unimpl\": {},\n  \
+             \"draws\": {},\n  \
+             \"swaps\": {},\n  \
+             \"unique_render_targets\": {},\n  \
+             \"shader_blobs_live\": {},\n  \
+             \"texture_cache_entries\": {}\n}}\n",
+            self.instructions,
+            self.imports,
+            self.unimpl,
+            self.draws,
+            self.swaps,
+            self.unique_render_targets,
+            self.shader_blobs_live,
+            self.texture_cache_entries,
+        )
+    }
 }
 
 fn escape_json_string(s: &str) -> String {