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>

hook/src/present_hook.hpp

@@ -0,0 +1,35 @@
+// Injected Present-hook video path: hooks IDXGISwapChain::Present in the target
+// game, copies the swapchain backbuffer into a shared keyed-mutex texture
+// (coop_video_<pid>), and publishes its dimensions/format to the host over IPC.
+// The host opens that texture by name and samples it -- a lower-latency, more
+// stable alternative to Windows Graphics Capture (which composites off the
+// desktop and can stutter / show a capture border).
+//
+// Only DXGI swapchains (D3D10/11/12-backed games) are caught; the backbuffer
+// must be an ID3D11Texture2D (the common D3D11 case). Games on D3D9 or a pure
+// D3D12 resource path won't engage this hook -- WGC stays as the fallback.
+#pragma once
+
+#include "ipc_client.hpp"
+
+namespace coop::hook
+{
+
+// Installs the Present hook. Grabs IDXGISwapChain::Present from a throwaway
+// swapchain and inline-hooks it, so every swapchain in the process is caught.
+// `ipc` must outlive the hook (used to publish the shared-texture descriptor).
+// Returns true once the hook is in place; safe to call repeatedly.
+bool install_present_hooks(IpcClient& ipc);
+
+// Removes the Present hook and releases the shared texture (best effort).
+void remove_present_hooks();
+
+// --- Diagnostics (used by the self-test) -----------------------------------
+
+// Cumulative Present() detours observed.
+std::uint64_t present_calls();
+
+// Frames actually copied into the shared texture (a backbuffer we could share).
+std::uint64_t present_frames_shared();
+
+} // namespace coop::hook