[conrade-ctc]

This adds experimental support to debug JIT-compiled C++ at the source level with gdb, from Python and from VS Code. It is off by default: only a build with -DCPPJIT_ENABLE_DEBUGGER_SUPPORT=ON has it. Without the option, the build has no new code, exported symbols or package files, and python/cppjit does not change.

In a build with the support, CPPJIT_DEBUGGER_SUPPORT=1 starts a debug session:

• The JIT compiles with -g -O0. With Write interpreter inputs to files for debuggers (experimental) CppInterOp#1156, each C++ input goes to a file, so gdb shows the source and binds file:line breakpoints, also in headers that cppjit.include loads.
• cppjit.debugger adds Python line breakpoints and steps through sys.monitoring.
• A gdb extension (cppjit/debugger/gdb) shows the Python frames between the C++ frames, with the variables of both. It reads CPython's _Py_DebugOffsets, so CPython needs no debug information (3.13 or later).
• examples/vscode-gdb uses it from VS Code through gdb's DAP server (gdb 14 or later). A small DAP proxy adds Python breakpoints, and steps between Python and C++.

Notes for review:

• The C++ changes are behind CPPJIT_WITH_DEBUGGER_SUPPORT: a hook pair around each JIT call in WrapperCall, a few exported variables for the debugger, and the import of cppjit.debugger in the module init.
• The gdb extension also reads a few layouts that the debug offsets do not give: frame globals, dict entries, and the CPPInstance fields. I think that is acceptable for an experimental feature, but a change of these layouts needs an update here.
• CI does not build with the option, so the gdb tests in test/test_debugging.py skip there. The two decoder tests run in CI from the source tree. Locally (Linux, gdb 16.3, CPython 3.13), all 21 tests pass with the option. I can add a CI cell with the option if you want one.
• file:line breakpoints need the CppInterOp change and a pin that has it. Breakpoints by function name work without it.

🤖 Done with the help of Claude Code (Opus 5.5, human in the loop)

[aaronj0]

Nice! So that your changes are tested on the CI, can you add a commit like cc7edf7 pointing to your CppInterOp remote and branch? That would stack your changes and any iterations on compiler-research/CppInterOp#1156 will automatically reflect here