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/mkb_ring_test.cpp

@@ -0,0 +1,82 @@
+// Unit test for the MKB event ring (SPSC push/pop, wrap-around, full/empty).
+#include <cstdio>
+
+#include "coop/protocol.hpp"
+
+using namespace coop;
+
+namespace
+{
+int g_failures = 0;
+void check(bool cond, const char* what)
+{
+	if (!cond)
+	{
+		std::printf("FAIL: %s\n", what);
+		++g_failures;
+	}
+}
+} // namespace
+
+int main()
+{
+	MkbRing ring{};
+
+	// Empty pop fails.
+	MkbEvent out{};
+	check(!pop_mkb_event(ring, out), "pop on empty ring returns false");
+
+	// Push then pop returns the same event, FIFO.
+	for (std::uint32_t i = 0; i < 10; ++i)
+	{
+		MkbEvent ev{Mkb_KeyDown, i, static_cast<int>(i) * 2, static_cast<int>(i) * 3};
+		check(push_mkb_event(ring, ev), "push succeeds with room");
+	}
+	for (std::uint32_t i = 0; i < 10; ++i)
+	{
+		check(pop_mkb_event(ring, out), "pop succeeds with data");
+		check(out.code == i && out.x == static_cast<int>(i) * 2 && out.y == static_cast<int>(i) * 3,
+		      "popped event matches pushed (FIFO)");
+	}
+	check(!pop_mkb_event(ring, out), "ring empty again after draining");
+
+	// Fill to capacity, then one more push is dropped.
+	for (std::uint32_t i = 0; i < kMkbQueueSize; ++i)
+	{
+		check(push_mkb_event(ring, MkbEvent{Mkb_Char, i, 0, 0}), "push fills to capacity");
+	}
+	check(!push_mkb_event(ring, MkbEvent{Mkb_Char, 999, 0, 0}), "push on full ring is dropped");
+
+	// Drain and verify order survived a full buffer.
+	for (std::uint32_t i = 0; i < kMkbQueueSize; ++i)
+	{
+		check(pop_mkb_event(ring, out) && out.code == i, "full-buffer drain is in order");
+	}
+
+	// Wrap-around: indices are free-running, so many cycles must keep working.
+	std::uint32_t produced = 0, consumed = 0;
+	for (int cycle = 0; cycle < 1000; ++cycle)
+	{
+		for (int k = 0; k < 50; ++k)
+		{
+			if (push_mkb_event(ring, MkbEvent{Mkb_MouseDown, produced, 0, 0}))
+			{
+				++produced;
+			}
+		}
+		while (pop_mkb_event(ring, out))
+		{
+			check(out.code == consumed, "wrap-around preserves FIFO order");
+			++consumed;
+		}
+	}
+	check(produced == consumed, "all wrap-around events consumed");
+
+	if (g_failures == 0)
+	{
+		std::printf("PASS: mkb_ring_test\n");
+		return 0;
+	}
+	std::printf("FAIL: %d checks failed\n", g_failures);
+	return 1;
+}