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>

tests/CMakeLists.txt

@@ -20,6 +20,17 @@ add_executable(audio_ring_test audio_ring_test.cpp)
 target_link_libraries(audio_ring_test PRIVATE coop_common)
 add_test(NAME audio_ring_test COMMAND audio_ring_test)
 
+# Unit test for the MKB event ring (SPSC push/pop, wrap-around, full/empty).
+add_executable(mkb_ring_test mkb_ring_test.cpp)
+target_link_libraries(mkb_ring_test PRIVATE coop_common)
+add_test(NAME mkb_ring_test COMMAND mkb_ring_test)
+
+# Unit test for the host->game mouse coordinate mapping (letterbox inverse +
+# decorated-window client offset). Header-only, no device.
+add_executable(mkb_map_test mkb_map_test.cpp)
+target_include_directories(mkb_map_test PRIVATE ${CMAKE_SOURCE_DIR}/host/src)
+add_test(NAME mkb_map_test COMMAND mkb_map_test)
+
 # Integration test for WASAPI process-loopback capture. Reuses the shipping
 # capture code and captures from coop_tone (a known sine-wave render process).
 add_executable(audio_loopback_test
@@ -114,6 +125,8 @@ add_test(NAME opengl_hook_test COMMAND opengl_hook_test)
 coop_output_subdir(tests
 	hook_selftest
 	audio_ring_test
+	mkb_ring_test
+	mkb_map_test
 	audio_loopback_test
 	audio_hook_test
 	srgb_format_test