[client] transport: introduce pluggable frame transports

Move shared-memory ownership, LGMP session handling, queue access, and
DMA setup behind a transport interface. The LGMP backend preserves the
existing zero-copy frame and DMA paths while owning its lgmp:* options.

Expose the initialized EGL context through a versioned renderer interop
record for future accelerated decode backends. Add an LGMP-independent,
deterministic test transport for graphics-pipeline validation.

client/transports/LGMP/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.10)
+project(transport_LGMP LANGUAGES C)
+
+add_library(transport_LGMP STATIC
+  lgmp.c
+)
+
+target_link_libraries(transport_LGMP
+  lg_common
+  lgmp
+)