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>

host/src/main.cpp

@@ -18,6 +18,7 @@
 #include "controllers_panel.hpp"
 #include "d3d11_window.hpp"
 #include "imgui_layer.hpp"
+#include "inject/mkb_forward.hpp"
 #include "injection_panel.hpp"
 #include "input/xinput_source.hpp"
 #include "log_panel.hpp"
@@ -194,6 +195,10 @@ int run()
 		}
 		coop::apply_layout_end_frame(); // clear the one-shot "Reset layout" force
 
+		// Forward the host window's mouse/keyboard into the game (when the MKB
+		// subsystem is on, we're focused, and ImGui isn't using the event).
+		coop::forward_mkb_frame(injection, window.hwnd(), capture.mirroring(), capture.source_hooked());
+
 		RECT client = {};
 		GetClientRect(window.hwnd(), &client);
 		const auto dst_w = static_cast<std::uint32_t>(client.right - client.left);