Phase 2: Present-hook video path (shared-texture mirror)

Add an injected IDXGISwapChain::Present / Present1 hook as a lower-latency,
border-free alternative to WGC. The hook copies the swapchain backbuffer into a
shared keyed-mutex texture (coop_video_<pid>); the host opens it by name and
samples it. New opt-in HookSubsys_Video (protocol v6 -> v7); the Video mirror
panel gains a WGC vs Hooked source toggle that installs/removes the subsystem.

Verified by present_hook_test (drives a real D3D11 swapchain end-to-end and reads
the rendered pixels back through the shared texture) and against Phantom Brave
(D3D9: hook installs cleanly and stays idle, WGC fallback). All 5 tests pass.

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

common/include/coop/protocol.hpp

@@ -11,7 +11,7 @@ namespace coop
 
 // Bump whenever the layout of SharedBlock or CoopPadState changes. The hook
 // refuses to attach to a host with a mismatched version.
-inline constexpr std::uint32_t kProtocolVersion = 6;
+inline constexpr std::uint32_t kProtocolVersion = 7;
 
 // 'COOP' little-endian, used to sanity-check the mapping before trusting it.
 inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
@@ -66,7 +66,8 @@ enum HookSubsystem : std::uint32_t
 	HookSubsys_Input = 0, // XInput hooks (forward the guest pad)
 	HookSubsys_Focus = 1, // focus spoof (keep the game running unfocused)
 	HookSubsys_Audio = 2, // WASAPI render-hook (audio mirror without echo)
-	HookSubsys_Count = 3,
+	HookSubsys_Video = 3, // IDXGISwapChain::Present hook (shared-texture video mirror)
+	HookSubsys_Count = 4,
 };
 
 // Maximum individual hooks reported in the registry (a few per subsystem).
@@ -134,6 +135,29 @@ struct HookControl
 	std::atomic<std::uint32_t> subsystem_disabled[HookSubsys_Count];
 };
 
+// Present-hook video channel. When the video subsystem is installed, the hook
+// copies the game's swapchain backbuffer into a shared keyed-mutex texture named
+// coop_video_<pid> and publishes its dimensions/format here; the host opens that
+// texture by name and samples it (a lower-latency alternative to WGC). The hook
+// is the sole writer. `generation` bumps on every published frame (0 = nothing
+// shared yet); width/height/format describe the currently shared texture, so the
+// host reopens it whenever they change. The keyed mutex uses key 0 on both sides.
+struct VideoShare
+{
+	std::atomic<std::uint32_t> generation; // bumps per published frame; 0 = none yet
+	std::uint32_t width;                   // shared texture dimensions / DXGI format
+	std::uint32_t height;
+	std::uint32_t format;                  // DXGI_FORMAT of the shared texture
+	std::uint64_t present_calls;           // cumulative Present() detours (diagnostic)
+};
+
+// The shared backbuffer texture is named per target pid, like the audio ring.
+inline constexpr wchar_t kVideoSharePrefix[] = L"Local\\coop_video_";
+
+// Keyed-mutex key both producer and consumer use (a plain cross-process mutex on
+// the texture; the keyed mutex is created released at key 0).
+inline constexpr std::uint64_t kVideoMutexKey = 0;
+
 // Top-level shared block. The host is the sole writer of pad state; the hook is
 // the sole reader. A seqlock (even = stable, odd = write in progress) lets the
 // reader grab a torn-free snapshot without a kernel lock on the hot path.
@@ -151,8 +175,8 @@ struct SharedBlock
 	// Host -> hook control (which subsystems to install).
 	HookControl control;
 
-	// Phase 2 appends the shared-texture handle/dimensions control fields here;
-	// keep new members at the end so existing offsets never shift.
+	// Hook -> host Present-hook video channel (shared-texture dimensions/format).
+	VideoShare video;
 };
 
 static_assert(std::atomic<std::uint32_t>::is_always_lock_free,