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>

host/src/capture/shared_texture.hpp

@@ -0,0 +1,69 @@
+// Host-side consumer of the injected Present-hook's shared backbuffer texture.
+// Opens the keyed-mutex texture the hook publishes (coop_video_<pid>) by name,
+// copies the latest frame into a private texture under the keyed mutex, and
+// exposes an SRV the FrameRenderer can draw -- the lower-latency alternative to
+// Windows Graphics Capture. See hook/src/present_hook.cpp and coop/protocol.hpp.
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+#include <d3d11_1.h>
+#include <wrl/client.h>
+
+#include "ipc/ipc_server.hpp"
+
+namespace coop
+{
+
+class SharedTextureSource
+{
+public:
+	// Binds to the host's device (must support ID3D11Device1). Returns false if not.
+	bool init(ID3D11Device* device);
+
+	// Polls the hook's video channel for `pid`. (Re)opens the shared texture when
+	// the pid/dimensions/format change, and copies the newest frame into the
+	// private texture under the keyed mutex. Returns true if a frame is available
+	// to draw (sticky: keeps the last copy until a new one or a reset).
+	bool update(const VideoShareView& share, unsigned long pid);
+
+	// Drop the opened resources (target changed / mirror turned off).
+	void reset();
+
+	[[nodiscard]] ID3D11ShaderResourceView* srv() const
+	{
+		return srv_.Get();
+	}
+	[[nodiscard]] std::uint32_t width() const
+	{
+		return width_;
+	}
+	[[nodiscard]] std::uint32_t height() const
+	{
+		return height_;
+	}
+	[[nodiscard]] std::uint64_t frames_copied() const
+	{
+		return frames_copied_;
+	}
+
+private:
+	bool reopen(unsigned long pid, const VideoShareView& share);
+
+	Microsoft::WRL::ComPtr<ID3D11Device1> device_;
+	Microsoft::WRL::ComPtr<ID3D11DeviceContext> ctx_;
+	Microsoft::WRL::ComPtr<ID3D11Texture2D> shared_; // opened from the hook, by name
+	Microsoft::WRL::ComPtr<IDXGIKeyedMutex> mutex_;
+	Microsoft::WRL::ComPtr<ID3D11Texture2D> private_; // our sampled copy
+	Microsoft::WRL::ComPtr<ID3D11ShaderResourceView> srv_;
+
+	unsigned long pid_ = 0;
+	std::uint32_t width_ = 0;
+	std::uint32_t height_ = 0;
+	std::uint32_t format_ = 0;
+	std::uint32_t last_generation_ = 0;
+	std::uint64_t frames_copied_ = 0;
+};
+
+} // namespace coop