Move the stub generator to a root scripts/ folder #71

The maintainer's decision (2026-10-03): pythonx/_build/ moves to a root scripts/, a new top-level entry the maintainer approved. Done when scripts/gen_stubs.py works with every reference updated (AGENTS.md root list and path table, PROJECT.md, SPEC, tests/test_stubs.py, the stubs' header line), CI passes, and a test shows the wheel carries no scripts/.