Fix cross-platform wheel CI tagging

[zhoubot]

Summary

• fix the wheel packaging path so CI always emits deterministic platform tags on Linux/macOS instead of inheriting the host macOS version
• move wheel platform tagging into the shared wheel builder and add a generic helper for cross-platform validation
• re-enable manual CI dispatch support so GitHub Actions can be triggered immediately with gh workflow run

Decisions

• Implements 0013/0014

Validation

• python3 packaging/wheel/get_wheel_plat_name.py --system macos --arch arm64
• python3 packaging/wheel/get_wheel_plat_name.py --system macos --arch x86_64
• python3 packaging/wheel/get_wheel_plat_name.py --system linux --arch x86_64
• python3 -m py_compile packaging/wheel/create_wheel.py packaging/wheel/get_wheel_plat_name.py packaging/wheel/platform_tags.py compiler/frontend/pycircuit/packaged_toolchain.py compiler/frontend/pycircuit/cli.py
• python3 -m venv .venv-wheel-build && . .venv-wheel-build/bin/activate && pip install setuptools wheel && python packaging/wheel/create_wheel.py --install-dir "$PWD/.pycircuit_out/toolchain/install" --out-dir "$PWD/dist-cifix"
• python3 -m venv .venv-wheel-ci && . .venv-wheel-ci/bin/activate && pip install dist-cifix/*.whl && PYC_USE_INSTALLED_PYTHON_PACKAGE=1 PYC_TOOLCHAIN_ROOT="$(python -c 'import pathlib, pycircuit; print((pathlib.Path(pycircuit.__file__).resolve().parent / "_toolchain").as_posix())')" bash flows/scripts/run_examples.sh

Notes

• The macOS wheel now defaults to macosx_11_0_arm64 on arm64 and macosx_10_13_x86_64 on x86_64, instead of using the runner host version in the wheel tag.
• Upstream GitHub workflows were in disabled_manually state; this PR re-enables CI triggering via workflow_dispatch on .github/workflows/ci.yml.