Native C++ pose-graph-optimization (PGO) for DimOS loop closure — GTSAM iSAM2 + PCL ICP, with Scan-Context descriptors and AprilTag landmark factors.

Extracted from the DimOS nav stack so the C++ and its nix flake can be built and pinned independently of the Python tree.

nix build .#default        # -> result/bin/pgo

Outputs pgo (the live module binary) and pgo_landmark_test (a no-LCM lockstep unit test driving SimplePGO directly).

• main.cpp — LCM I/O loop, wires odometry/lidar/tags into the optimizer
• simple_pgo.{cpp,h} — iSAM2 pose graph + ICP loop closure
• scan_context.{cpp,h} — Scan-Context place descriptors / loop candidate search
• commons.{cpp,h}, point_cloud_utils.hpp — shared helpers
• dimos_native_module.hpp — DimOS NativeModule LCM glue
• msgs/*.hpp — C++ wire helpers mirroring dimos.navigation.jnav.msgs.* (Graph3D, GraphDelta3D, Landmark); keep encode() in sync with the Python decoders.

dimos/navigation/jnav/components/loop_closure/gsc_pgo/module.py, via nix build "github:jeff-hykin/gsc_pgo#default".