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/shared_memory.hpp

@@ -128,4 +128,11 @@ inline std::wstring shared_memory_name(unsigned long target_pid)
 	return std::wstring(kSharedMemoryPrefix) + std::to_wstring(target_pid);
 }
 
+// Name of the Present-hook shared backbuffer texture (created by the hook with
+// CreateSharedHandle, opened by the host with OpenSharedResourceByName).
+inline std::wstring video_share_name(unsigned long target_pid)
+{
+	return std::wstring(kVideoSharePrefix) + std::to_wstring(target_pid);
+}
+
 } // namespace coop