Add mouse + keyboard forwarding (MKB subsystem, opt-in)

Forward the host window's clicks and keystrokes into the injected game so guests
can drive menus / "Press Start" / text entry that a pad can't.

Protocol (v7->v8): new HookSubsys_Mkb and an SPSC MkbRing of MkbEvents in
SharedBlock (host produces, hook consumes); push/pop helpers.

Hook (hook/src/mkb_hook.cpp, new subsystem): a worker-loop pump drains the ring at
~5 ms and PostMessageW's the matching window messages (WM_KEY*/WM_CHAR, mouse
buttons, WM_MOUSEWHEEL) to the game's main window; it also inline-hooks user32
GetAsyncKeyState / GetKeyboardState / GetCursorPos (stdcall trampolines per the x86
rule) to report a synthesized state so polling games react too. Removing the
subsystem clears all synthesized keys (no stuck input).

Host: the Injection panel gets a "Mouse + keyboard forwarding" subsystem toggle
(opt-in, default off -- the toggle is the hook). host/src/inject/mkb_forward.cpp
reads ImGui IO each frame and forwards only when the host window is focused and
ImGui isn't capturing the event; keyboard always, mouse only while mirroring (clicks
+ wheel, not movement). Mouse coords are mapped through the letterbox to game-client
space (host/src/inject/mkb_map.hpp), accounting for WGC-of-decorated-window vs
hooked/borderless. RawInput/DirectInput games are out of scope for this version.

Verified: new mkb_ring_test + mkb_map_test pass; full build x64 + x86 clean; ctest
x64 9/9 and x86 3/3 green (no regression from the protocol bump). The subsystem is
opt-in, so it can't affect existing behavior unless enabled; the end-to-end
click-into-game path needs live Remote Play + a real game to confirm.

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

tools/audio_probe/main.cpp

@@ -308,12 +308,12 @@ int wmain(int argc, wchar_t** argv)
 	}
 
 	// Dump the hook registry so the installed-hooks list can be verified headless.
-	static const char* kSubsys[] = {"Input", "Focus", "Audio", "Video"};
+	static const char* kSubsys[] = {"Input", "Focus", "Audio", "Video", "MKB"};
 	std::printf("\nInstalled hooks (%u):\n", status.hook_entry_count);
 	for (std::uint32_t i = 0; i < status.hook_entry_count && i < coop::kMaxHookEntries; ++i)
 	{
 		const coop::HookEntry& e = status.hook_entries[i];
-		std::printf("  [%-5s] %-34s %s  calls=%llu\n", e.subsystem < 4 ? kSubsys[e.subsystem] : "?", e.name,
+		std::printf("  [%-5s] %-34s %s  calls=%llu\n", e.subsystem < 5 ? kSubsys[e.subsystem] : "?", e.name,
 		            e.installed ? "ON " : "off", static_cast<unsigned long long>(e.calls));
 	}