Fix stale removal comments (persistent, not destroy) + add deterministic install test

- The remove_* comments still described the superseded "disable -> drain -> destroy"
  flow; the code keeps hooks alive (persistent) and re-enables on re-install. Updated
  the comments to match, and corrected the XInput note (its detours return synthesized
  state and never call the trampoline, so destroying its vector is safe -- unlike the
  trampoline-calling present/MKB/focus-cursor hooks).
- hook_install_test: a fast, single-threaded contract test for hook_install.hpp --
  install_inline creates the hook once and reuses the SAME trampoline across 50
  install/remove cycles (never freed -> no stale-detour UAF), toggling enable/disable
  cleanly. Fills the guard the removed (flaky, concurrency-bound) reproducer left, with
  no threads so it can't flake on SafetyHook's enable/disable atomicity.

x64 23/23, x86 3/3.

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

tests/hook_install_test.cpp

@@ -0,0 +1,105 @@
+// Deterministic, single-threaded contract test for the PERSISTENT inline-hook model
+// (hook/src/hook_install.hpp + hook_guard.hpp): install_inline() creates a hook once and thereafter
+// only re-enables it, so the trampoline is allocated ONCE and never freed across install/remove
+// cycles -- which is what makes spamming a subsystem on/off safe (a stale detour can never jump
+// through a freed trampoline). The mock_game_test storm covers the concurrent behaviour; this guards
+// the contract with no threads, so it can't flake on SafetyHook's enable/disable atomicity.
+#include <cstdint>
+#include <cstdio>
+
+#include <safetyhook.hpp>
+
+#include "hook_guard.hpp"
+#include "hook_install.hpp"
+
+using coop::hook::DetourGate;
+
+namespace
+{
+int g_failures = 0;
+void check(bool ok, const char* what)
+{
+	std::printf("%s %s\n", ok ? "  ok:" : "FAIL:", what);
+	if (!ok)
+	{
+		++g_failures;
+	}
+}
+
+safetyhook::InlineHook g_hook;
+DetourGate g_gate;
+
+// A target with a real (relocatable) body, never inlined.
+__declspec(noinline) int target_fn(int x)
+{
+	volatile int a = x;
+	a = a * 3 + 7;
+	a ^= (a >> 2);
+	a += (a << 1);
+	return a;
+}
+
+constexpr int kMark = 100000; // the detour adds this so "did the detour run" is observable
+
+int detour_fn(int x)
+{
+	DetourGate::Guard guard(g_gate);
+	return g_hook.call<int>(x) + kMark; // reach the original through the trampoline
+}
+
+void install()
+{
+	coop::hook::install_inline(g_hook, reinterpret_cast<void*>(&target_fn), reinterpret_cast<void*>(&detour_fn));
+}
+void remove()
+{
+	if (g_hook && !g_hook.disable())
+	{
+		// surface, don't discard
+	}
+	g_gate.drain(); // persistent model: disable + drain, but do NOT destroy
+}
+} // namespace
+
+int main()
+{
+	const int probe = 5;
+	const int original = target_fn(probe); // unhooked baseline
+
+	// First install: hook enabled, detour runs and reaches the original through the trampoline.
+	install();
+	check(g_hook.enabled(), "install_inline enables the hook");
+	void* const tramp1 = g_hook.original<void*>();
+	check(tramp1 != nullptr, "trampoline allocated");
+	check(target_fn(probe) == original + kMark, "detour runs and reaches the original via the trampoline");
+
+	// Remove (persistent): disable + drain, but keep the hook ALIVE.
+	remove();
+	check(!g_hook.enabled(), "remove disables the hook");
+	check(static_cast<bool>(g_hook), "remove keeps the hook alive (persistent -- not destroyed)");
+	check(target_fn(probe) == original, "disabled -> the original runs");
+
+	// Re-install: SAME trampoline reused (not recreated/freed), detour runs again.
+	install();
+	check(g_hook.enabled(), "re-install re-enables the hook");
+	check(g_hook.original<void*>() == tramp1, "re-install REUSES the same trampoline (never freed -> no UAF)");
+	check(target_fn(probe) == original + kMark, "detour runs again after re-enable");
+
+	// Many cycles: the trampoline must never change and enable/disable must toggle cleanly.
+	bool reuse = true, toggles = true;
+	for (int i = 0; i < 50; ++i)
+	{
+		remove();
+		toggles = toggles && !g_hook.enabled();
+		install();
+		reuse = reuse && (g_hook.original<void*>() == tramp1);
+		toggles = toggles && g_hook.enabled();
+	}
+	check(reuse, "trampoline stays identical across 50 install/remove cycles (no churn, no leak)");
+	check(toggles, "enable/disable toggles cleanly across the cycles");
+	check(target_fn(probe) == original + kMark, "hook still works after the cycles");
+
+	g_hook = {}; // teardown (process is single-threaded here, so destroying is fine)
+	std::printf(g_failures == 0 ? "PASS hook_install_test\n" : "FAILED hook_install_test (%d)\n", g_failures);
+	return g_failures == 0 ? 0 : 1;
+}