DX12 capture: fence the copy off the game's queue + add drop detection

Resolves the DX12 mirror stutter and makes dropped frames observable.

Decouple the D3D11On12 copy from the game's present queue. Submitting the
copy on the game's own present queue (the prior approach) ordered it
correctly but stalled the game's presents: GPU back-pressure, plus the
shared keyed-mutex AcquireSync is a CPU-blocking call on the render
thread. Running it on an independent queue avoids the stall but races the
game's render -> stale frames. Do both: run the copy on our own queue and
order it after the frame with an ID3D12Fence the game's present queue
signals (near-free) and our queue waits on. The present queue is still
recovered for late injection via the ExecuteCommandLists hook (now used to
signal the fence, not host the copy). Producer AcquireSync stays
non-blocking (timeout 0) so a busy mutex drops a mirror frame instead of
stalling the game.

Add drop detection (protocol v12 -> v13). The hook counts captures skipped
because the keyed mutex was busy (VideoShare.frames_dropped); the host
counts published frames it never displayed (generation gaps). The Video
panel shows "Frames lost: N/s capture  N/s display", red when nonzero.
This confirmed the game-window-vs-mirror behavior is a display-path
artifact (unfocused windows lose VRR/independent flip), not a capture loss.

Add a one-shot present-pattern log: per distinct swapchain (size/format/
buffer index) and per distinct present-flags value, with DXGI_PRESENT_TEST
spelled out as an occlusion probe that draws nothing -- which is why
Miles Morales shows ~2 presents per captured frame (the test present is
counted but produces no frame).

Docs: add the DX12 capture lessons to the README (rotating back buffer,
fence/own-queue, capture-at-Present decoupling from DWM) and drop the now
-moot DX12 overhead future-work item.

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

common/include/coop/protocol.hpp

@@ -12,7 +12,7 @@ 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 = 12;
+inline constexpr std::uint32_t kProtocolVersion = 13;
 
 // 'COOP' little-endian, used to sanity-check the mapping before trusting it.
 inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
@@ -168,6 +168,9 @@ struct VideoShare
 	std::uint32_t format;                  // DXGI_FORMAT of the shared texture
 	std::uint64_t present_calls;           // cumulative Present() detours (diagnostic)
 	std::int64_t present_qpc;              // QueryPerformanceCounter at the last publish
+	std::uint64_t frames_dropped;          // cumulative captures skipped because the shared
+	                                       // keyed mutex was busy (host mid-copy) -- a frame
+	                                       // the game produced that never reached the mirror
 };
 
 // --- Mouse + keyboard forwarding -------------------------------------------