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>

hook/src/mkb_hook.hpp

@@ -0,0 +1,23 @@
+// Mouse + keyboard forwarding subsystem (HookSubsys_Mkb).
+//
+// The host captures its own window's MKB input and pushes events into the shared
+// MkbRing. This subsystem drains them on the worker thread, posts the matching
+// window messages to the game's main window (so message-driven games react), and
+// maintains a synthesized input state that hooked GetAsyncKeyState /
+// GetKeyboardState / GetCursorPos report (so polling games react too). The toggle
+// is the subsystem itself: not installed -> nothing is forwarded.
+#pragma once
+
+#include "ipc_client.hpp"
+
+namespace coop::hook
+{
+
+bool install_mkb_hooks(IpcClient& ipc);
+void remove_mkb_hooks();
+
+// Drain the host's MKB event queue: post window messages + update synthesized
+// state. Called frequently from the worker loop while the subsystem is installed.
+void mkb_pump(IpcClient& ipc);
+
+} // namespace coop::hook