Move src/core/python to src/python/packaging

[jiwenc-nv]

Follow-up to #842, deliberately deferred from it.

Why

After #842, src/core/python/ contains no package source. Everything in it is wheel machinery:

pyproject.toml.in, setup.py, MANIFEST.in	classic wheel build
generate_stubs.py, stubgen_pyproject.toml	stub generation
8× requirements*.txt	dependency source of truth
CMakeLists.txt	staging install rule, python_wheel target, stubgen

(setup.py and generate_stubs.py are .py, but build tools — neither ships.)

Two problems with leaving it:

1. src/python/ and src/core/python/ now mean opposite things — package source vs wheel machinery. Same name, one nested under the other's parent. A newcomer greps one and finds the other.
2. It sits under src/core/, which is otherwise a list of C++ modules.

Proposal

Move the directory wholesale to src/python/packaging/.

Preferred over a repo-root packaging/ because it packages src/python/isaacteleop/ — locality is real — and it keeps everything Python-distribution-related under one root, matching the repo convention of sources under src/.

Move as a unit. Splitting would strand the requirements*.txt away from the pyproject.toml.in that reads them via file = [...].

Safe from leaking into either wheel, verified: the staging glob is scoped to isaacteleop/*.py (src/python/CMakeLists.txt:36) and wheel.packages maps only isaacteleop (pyproject.toml:135), so a sibling directory under src/python/ cannot enter the staged tree or either wheel.

Blast radius — smaller than it looks

15 references to src/core/python outside itself. 13 are prose comments; only these are load-bearing:

• src/viz/python/CMakeLists.txt:43 — ${CMAKE_SOURCE_DIR}/src/core/python/stubgen_pyproject.toml
• src/viz/python/CMakeLists.txt:47 — ${CMAKE_SOURCE_DIR}/src/core/python/generate_stubs.py
• src/core/CMakeLists.txt:45 — add_subdirectory(python)

Worth noting the comment hits in .github/workflows/build-ubuntu.yml:100 and .github/actions/setup-v2d-src/action.yml:8 are explanatory text, not paths — an earlier estimate treated them as CI breakage risk and over-scoped the change on that basis.

The 13 comments should still be updated, in pyproject.toml (3), src/viz/python/CMakeLists.txt (2), src/python/CMakeLists.txt (2), src/core/retargeting_engine_tests/python/pyproject.toml (2), and one each in src/core/AGENTS.md, scripts/setup_v2d_src.sh, src/core/retargeting_engine_tests/python/CMakeLists.txt, and the two CI files above.

Why it was deferred

Bisectability. Bundling a directory rename with #842's ~95-file move means a CI failure can't be attributed to either half.

Verification

Same gates as #842 — the change should be provably inert:

• staged tree (python_package/<config>/isaacteleop/) byte-identical by sha256
• classic wheel members and hashes unchanged
• cmake --install --component isaacteleop_wheel still yields zero .py
• ctest green