refactor(geometry): embed C++ METIS implementation

[KemengHuang]

Summary

• build the private C++ METIS/GKlib subset as uipc_metis in both CMake and XMake
• remove the legacy external/METIS and external/GKlib source trees and targets
• replace the unused LGPL/BSD compatibility pieces with license-clean C++/runtime facilities while preserving legacy equal-key ordering
• harden mesh_partition against zero size, 32-bit capacity overflow, invalid IDs, and retry underflow
• preserve METIS/GKlib Apache-2.0 attribution and document the architecture decision

Validation

• clean CMake configure and independent CMake/XMake uipc_metis builds
• CMake and XMake geometry suites: 38 cases / 270 assertions each
• repository script tests: 49/49; CTest fast: 3/3
• full Doxygen/MkDoxy/MkDocs build
• Linux ASan + UBSan on the 13,328-part fluffy_ball case
• old C vs new C++ byte-for-byte and per-vertex parity on Windows/MSVC and Linux/GCC:
  • fluffy_ball.msh: maximum sizes 256, 16, 8, and 4
  • animal_well.msh: maximum sizes 8 and 4
  • zero differing vertex partition IDs in every comparison
• all ten retained sort wrappers: 1,440 adversarial/boundary calls, identical 11,374,128-byte output on MSVC and GCC