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/golden/sylpheed_n50m.json

@@ -0,0 +1,10 @@
+{
+  "instructions": 50000008,
+  "imports": 407415,
+  "unimpl": 0,
+  "draws": 0,
+  "swaps": 2,
+  "unique_render_targets": 0,
+  "shader_blobs_live": 0,
+  "texture_cache_entries": 0
+}