IoUringAssmEngn: map StagingBuff w/mmap; reg w/io_uring; add eventFd

StagingBuffer:
We now allocate memory with mmap(MAP_ANONYMOUS) so that we can be
sure it can be pinned with io_uring_register_buffers(). This
ensures that if DMA is possible, it should be usable.

	IoUringAssemblyEngine:
We now register an eventfd with io_uring so that we can listen
for CQEs with boost::asio.

stimBuffApis/livoxGen1/stagingBuffer.h

@@ -9,6 +9,9 @@
 #include <string>
 #include <sstream>
 #include <algorithm>
+#include <sys/mman.h>
+#include <sys/uio.h>
+#include <unistd.h>
 
 #include "frameAssemblyDesc.h"
 
@@ -96,6 +99,18 @@ public:
 	void startAssembly() { assemblingFlag.store(true); }
 	void stopAssembly() { assemblingFlag.store(false); }
 
+	/**	EXPLANATION:
+	 * Returns an iovec for io_uring registration.
+	 * The buffer is mmap()-allocated and suitable for IORING_REGISTER_BUFFERS.
+	 */
+	struct iovec getIoUringRegisterIoVec() const
+	{
+		struct iovec iov;
+		iov.iov_base = buffer.get();
+		iov.iov_len = bufferNBytes;
+		return iov;
+	}
+
 	inline std::string stringify() const
 	{
 		std::ostringstream oss;
@@ -111,8 +126,25 @@ public:
 private:
 	void computeSlotStrideAndBufferSize();
 
-	// Buffer data
-	std::unique_ptr<uint8_t[]> buffer;
+	// Custom deleter for mmap-allocated buffer
+	struct MmapDeleter
+	{
+		size_t size;
+		MmapDeleter(size_t s) : size(s) {}
+		
+		void operator()(uint8_t* ptr) const
+		{
+			if (ptr != nullptr && size > 0)
+			{
+				munmap(ptr, size);
+			}
+		}
+	};
+
+	// Buffer data - mmap-allocated for io_uring registration
+	// Using unique_ptr<uint8_t, MmapDeleter> instead of array syntax
+	// since we have a custom deleter that knows the size
+	std::unique_ptr<uint8_t, MmapDeleter> buffer;
 	size_t bufferNBytes;
 
 	// Layout/invariants
@@ -135,7 +167,7 @@ inline StagingBuffer::StagingBuffer(
 	const InputEngineConstraints& inputEngineConstraints_,
 	const OutputEngineConstraints& /*outputEngineConstraints*/,
 	size_t nDgramsPerFrame)
-: buffer(nullptr), bufferNBytes(0),
+: buffer(nullptr, MmapDeleter(0)), bufferNBytes(0),
 nDgramsPerFrame(nDgramsPerFrame), slotStrideNBytes(0),
 inputConstraints(inputEngineConstraints_),
 assemblingFlag(false)
@@ -148,7 +180,23 @@ assemblingFlag(false)
 
 	computeSlotStrideAndBufferSize();
 
-	buffer = std::make_unique<uint8_t[]>(bufferNBytes);
+	/* Allocate buffer using mmap() for io_uring registration
+	 * MAP_ANONYMOUS | MAP_PRIVATE creates anonymous, non-file-backed memory
+	 */
+	void* mmapped = mmap(
+		nullptr, bufferNBytes,
+		PROT_READ | PROT_WRITE,
+		MAP_ANONYMOUS | MAP_PRIVATE,
+		-1, 0);
+
+	if (mmapped == MAP_FAILED)
+	{
+		throw std::runtime_error(std::string(__func__)
+			+ ": StagingBuffer: mmap() failed");
+	}
+
+	buffer = std::unique_ptr<uint8_t, MmapDeleter>(
+		static_cast<uint8_t*>(mmapped), MmapDeleter(bufferNBytes));
 	currentNBytes.store(0);
 
 	// Build FrameAssemblyDesc once