build: modernize Python packaging and linting (PEP 621, ruff, uv)

[philomath213]

Replaces the flake8/black lint stack with ruff + pre-commit, and moves packaging onto current standards.

• ruff replaces flake8 and black, run through pre-commit (which also owns clang-format); the Linter job moves to Python 3.14
• PEP 517/621: adds the missing [build-system] table — builds currently rely on pip's legacy fallback — moves metadata to [project], drops the distutils.version.LooseVersion import, adds MANIFEST.in
• Dev dependencies via uv: PEP 735 [dependency-groups] + uv.lock; removes requirements_dev.txt, requirements_dev.lock (it pinned a local --find-links path and was unusable) and setup.cfg
• Declares numpy as a runtime dependency — it was never declared, so a bare pip install tenseal produced an unimportable package
• Removes two dead CI scripts

Dropping black also closes both open Dependabot alerts, and is what unblocks linting on Python 3.14.

Verified: wheel builds end-to-end and imports, sdist carries CMakeLists.txt/cmake//.proto, twine check and pre-commit run --all-files clean.

The style: commit is a mechanical formatting pass, split out to keep the tooling diff reviewable.