Refactor jig_auto_package python install to use ament_python_install_package

[claudepad]

Background

jig_auto_package.cmake currently installs the per-package Python tree by hand:

• _jig_create_top_level_python_package writes a generated __init__.py and uses raw install(FILES …) to drop it under ${PYTHON_INSTALL_DIR}/${PROJECT_NAME}/.
• _jig_generate_python_node does the same for each node's user .py files and the codegen output, again with raw install(FILES …).

ament_cmake_python only registers the pythonpath.sh / pythonpath.dsv environment hook from inside ament_python_install_package / ament_python_install_module. Neither of those is called by the auto-package flow, so the hook never gets registered.

This was invisible on jazzy/kilted because there PYTHON_INSTALL_DIR resolves to lib/python${PYV}/site-packages, which ament_export_pythonpath adds to PYTHONPATH by default. On humble, PYTHON_INSTALL_DIR resolves to local/lib/python3.10/dist-packages (Debian Python convention), which the default exported pythonpath hook does not cover — so source install/setup.bash left the package off PYTHONPATH and every launched Python node failed with ModuleNotFoundError: No module named '<pkg>'.

What we did as a stopgap

_jig_create_top_level_python_package now calls the private idempotent macro _ament_cmake_python_register_environment_hook() after the manual install. This is the same macro that ament_python_install_package uses internally, and its definition is identical across humble/jazzy/kilted in ament_cmake_python-extras.cmake. Verified: humble dagger run no longer reports No module named 'jig_example'.

That patch is tactical — it relies on a private ament macro and leaves the bespoke install flow in place.

What this ticket is for

Replace the manual install(FILES …) calls in _jig_create_top_level_python_package and _jig_generate_python_node with a single ament_python_install_package(\${PROJECT_NAME} PACKAGE_DIR <staging>) call.

Sketch

1. Stage the assembled package tree under \${CMAKE_CURRENT_BINARY_DIR}/python_staging/\${PROJECT_NAME}/:
   • top-level generated __init__.py
   • <node>/ subdir per Python node containing user .py files (symlinked from source via file(CREATE_LINK … SYMBOLIC) to preserve --symlink-install ergonomics) and the codegen output (interface.py, parameters.py, _parameters.py, generated __init__.py).
2. Redirect the per-node codegen add_custom_command outputs to write directly into the staging dir.
3. Wrap the staging steps in an add_custom_target(jig_python_staging ALL DEPENDS …).
4. Drop every Python install(FILES …) call from the two macros. Keep the wrapper-executable install (install(PROGRAMS … DESTINATION lib/\${PROJECT_NAME})) — that part is fine.
5. Call ament_python_install_package(\${PROJECT_NAME} PACKAGE_DIR \${staging}/\${PROJECT_NAME}) once from jig_auto_package, before ament_auto_package.
6. Remove the _ament_cmake_python_register_environment_hook() call added as the stopgap.

Why it's worth doing

• Drops a dependency on a private _-prefixed ament macro (_ament_cmake_python_register_environment_hook). It's stable today but isn't API.
• Aligns with the canonical ament path: setuptools-driven install, env hook auto-registered, future-distro-proof.
• Reduces the surface area of bespoke CMake in the macro.
• Sets us up to use SETUP_CFG / version metadata if we ever want to publish these as real wheels.

Acceptance criteria

• dagger call build-and-test --src=. --ros-distro=humble passes (modulo unrelated humble-rclpy compat issues being tracked separately).
• dagger call build-and-test --src=. --ros-distro=jazzy and …--ros-distro=kilted still pass.
• colcon build --symlink-install then editing a nodes/<node>/<node>.py file in the source tree takes effect on the next test run without a rebuild.
• share/<pkg>/environment/pythonpath.sh is generated for any package using jig_auto_package with Python nodes.
• The _ament_cmake_python_register_environment_hook call and its TODO(jig#NN) comment are removed.

Pointers

• Stopgap landed in jig/cmake/jig_auto_package.cmake (_jig_create_top_level_python_package macro).
• Reference impl: ament_cmake_python/cmake/ament_python_install_package.cmake (humble & jazzy share the structure).
• Branch where the bug surfaced: 23-humble-support (issue humble support #23).