base: add process memory metrics

[Force67]

Summary

• add an exported ProcessHandle and resident/peak memory query under base/process using mem_size and reference output
• support both current and external processes through Linux/Android pids, macOS Mach task ports, and Windows process handles
• wire PSAPI and consistent macOS platform defines into CMake/premake, including case-correct MinGW headers/libraries
• test current, invalid, and controlled external-process queries

Testing

• cmake -S . -B /tmp/opencode/equilibrium-process-memory-build -DEQ_BUILD_TESTS=ON
• cmake --build /tmp/opencode/equilibrium-process-memory-build --parallel 8
• /tmp/opencode/equilibrium-process-memory-build/base_unittests --gtest_filter=ProcessMetricsTest.* (3/3 passed)
• x86_64-w64-mingw32-g++ -std=c++23 -DBASE_IMPLEMENTATION -DOS_WIN -I. -Ibase -shared base/process/process_metrics_win.cc -lpsapi -o /tmp/opencode/process_metrics_win.dll
• full CTest passes all affected tests; the existing FunctionRef.CanInitializeWithCapturingLambda baseline test still fails