Preserve SafetyHook concurrency finding as a committed upstream repro

A multithreaded test that tight-looped InlineHook enable()/disable() while
other threads called the hooked function flaked ~1/10. Isolation proved this is
a SafetyHook limitation, not our code: with the hook created once (no install
race, no trampoline UAF), tight-loop toggling AVs ~1/3 of runs in Debug
(0xC0000005, faulting RIP in the target body), while a no-toggle control is
clean at ~60M calls. enable()/disable() re-patch the prologue in place under a
VEH page-trap that only relocates a thread parked ON the prologue; a thread in
the function body faults on the briefly-non-exec page and relies on instruction
retry, which under rapid toggling races a half-rewritten prologue.

Rather than silently drop the flaky test, preserve the finding:
- tools/sh_concurrency_repro/: minimal, committed, non-CI reproducer
  (coop_sh_concurrency_repro; --callonly is the control). Surfaces 5/16 AVs.
- docs/safetyhook-concurrency.md: upstream-ready write-up (mechanism + fix
  directions + why it does not affect us).
- README lessons-learned + memory updated; tests/CMakeLists cross-references it.

Our code stays in SafetyHook's safe envelope (install/remove reconciled from a
single tick-bounded worker thread, never a tight loop), so the mock_game_test
storm is reliably green; the persistent-trampoline contract is covered
deterministically by hook_install_test + detour_gate_test. Removes the temp
_sh_probe wiring.

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

CMakeLists.txt

@@ -139,6 +139,7 @@ if(COOP_BUILD_HOOK)
 	add_subdirectory(tools/audio_validate) # coop_audio_validate: quantify capture fidelity (pitch/SNR/clicks)
 	add_subdirectory(tools/input_probe) # coop_input_probe: inject + forward synthetic input
 	add_subdirectory(tools/vk_validate) # coop_vk_validate: validate the Vulkan backend vs a real game
+	add_subdirectory(tools/sh_concurrency_repro) # coop_sh_concurrency_repro: upstream SafetyHook repro (see docs/)
 	add_subdirectory(tests)
 endif()