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- 3rdparty/flatbuffers submodule pinned to the latest release tag v25.12.19 (7e163021). The runtime is header-only, so only 3rdparty/flatbuffers/include is ever used and no library is linked; CMake never calls add_subdirectory on it and flatc is not in the build graph (plan B section 8.1, inheriting the earlier plan's section 7.1).
- MobileGL/MG_Remote/Protocol/protocol.fbs carries the CONTROL PLANE only: SegmentRef, Hello, Welcome, CapsSnapshot, DefaultFramebufferInfo, SurfaceOp, SurfaceReply, ResyncRequest, ResyncDone, AuxRequest, Fatal, LogLine, union CtrlMsg and the CtrlEnvelope root with a file_identifier. Hot-path records are FlatBuffers structs generated from MG_Pipe/PipeCalls.def in a later package and are deliberately absent here, so record numbering never churns.
- Two deviations from the earlier plan's section 7.1 sketch, both deliberate: (a) ProgramReflection is not a union member - plan B ships program artifacts inside the create_shader_state CSO blob (section 8.2), and union tags are wire values that may only ever be appended, so reserving a tag for a message that may never exist is worse than appending one later; (b) maxComputeWorkGroupCount/Size are vectors, not [int:3] - fixed-size arrays are legal only in FlatBuffers structs, never in tables.
- scripts/gen_protocol.py resolves flatc as MOBILEGL_FLATC_EXECUTABLE, otherwise builds the pinned flatc ONCE into <repo>/../flatc-build (override with MOBILEGL_FLATC_BUILD_DIR), outside the project build graph. A flatc found on PATH is deliberately refused and a version mismatch against the pinned runtime is a hard error: the generated header static_asserts FLATBUFFERS_VERSION, so a stray flatc either fails to compile or churns the committed file on every machine. The earlier branch did the opposite - Protocol/CMakeLists.txt:22-38 add_subdirectory'd the FlatBuffers tree with FLATBUFFERS_BUILD_FLATC=ON whenever MOBILEGL_FLATC_EXECUTABLE was unset, which is exactly the NDK trap it claimed to avoid (cross-compile an arm64 flatc, then run it on the host).
- protocol_generated.h is committed with the project source header prepended by the generator, so regeneration is byte-identical: verified by running gen_protocol.py twice and by perturbing the file and regenerating it back.
- Reuse from Feat/CS-Delta-IPC: MobileGL/Protocol/mg_protocol_base.h, kept as the shared C vocabulary (result codes, byte spans, shm region, id typedefs) and keeping the structSize-first versioning discipline that section 14.2 calls the answer to risk B-R10. Dropped from it: MobileGLObjectKind / MobileGLObjectScope / MobileGLObjectHandle - plan B never puts GL object identity on the wire (the frontend allocates {slot, generation} handles in MG_Pipe, section 4.2.1), so a second identity vocabulary would be a drift surface with no reader. Added MOBILEGL_ERR_BUFFER_TOO_SMALL as an append-only code for the receive contract.