feat(kernel): KRNBUG-XEX-001 — real XexCheckExecutablePrivilege from XEX header bitmap

Replace stub_return_zero with a canary-faithful implementation that
returns bit `priv` of the loaded XEX's XEX_HEADER_SYSTEM_FLAGS
(key 0x00030000) bitmap. Mirrors xenia-canary
xboxkrnl_modules.cc:22-39: `(flags >> priv) & 1` for priv < 32, else 0.

Plumbing:
- xenia-xex: header_keys::SYSTEM_FLAGS const + get_system_flags() accessor.
- xenia-kernel/state.rs: pub xex_system_flags: u32 + xex_priv_logged
  HashSet for one-shot per-priv tracing.
- xenia-app: kernel.xex_system_flags wired in cmd_exec_inner.
- xenia-kernel/exports.rs: real export body + unit test covering
  bits 10/11/0/64 + zero-flags case.

Sylpheed's bitmap is 0x00000400 (only XEX_SYSTEM_PAL50_INCOMPATIBLE,
bit 10). At -n 500M with the fix:
- XGetAVPack: 0 -> 1 (priv-10 gate at lr=0x824ab598 flipped).
- 10 other canary-only exports + 9 producer PCs + 3 parked handles
  unchanged. Priv-11 site at sub_824A9710 is downstream and still
  not reached — AV/crypto block aborts after XGetAVPack returns
  our placeholder 0x16 (canary returns 8/HDMI; Sylpheed accepts
  only 3/4/6/8 per xenia-canary xam_info.cc:250-251).

Tests 588 -> 589. Lockstep deterministic (3 reruns identical):
n50m goes 50000008 -> 50000005 instr / 407415 -> 407417 imp / swaps=2 /
draws=0. Goldens re-baselined (sylpheed_n50m, sylpheed_n2m); oracle
test green.

Full chain-of-effects + next-frontier hand-off in audit-findings.md
under KRNBUG-XEX-001.

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

crates/xenia-app/tests/golden/sylpheed_n2m.json

@@ -1,16 +1,10 @@
 {
-  "path": "/home/fabi/RE Project Sylpheed/Project Sylpheed - Arc of Deception (USA, Europe) (En,Ja).iso",
   "instructions": 2000000,
-  "imports": 5634,
+  "imports": 5636,
   "unimpl": 0,
-  "packets": 0,
   "draws": 0,
   "swaps": 0,
-  "resolves": 0,
   "unique_render_targets": 0,
   "shader_blobs_live": 0,
-  "interrupts_delivered": 0,
-  "interrupts_dropped": 13,
-  "texture_cache_entries": 0,
-  "texture_decodes": 0
+  "texture_cache_entries": 0
 }

crates/xenia-app/tests/golden/sylpheed_n50m.json

@@ -1,6 +1,6 @@
 {
-  "instructions": 50000008,
-  "imports": 407415,
+  "instructions": 50000005,
+  "imports": 407417,
   "unimpl": 0,
   "draws": 0,
   "swaps": 2,