Tighten ruff config: enable F841, scope the F401 ignore to __init__.py

[johnnichev]

Tighten the ruff lint config

[tool.ruff.lint] selects F (pyflakes) but globally ignores F401 (unused imports) and F841 (unused local variables) in pyproject.toml. That's why dead locals and stray imports aren't caught in CI.

• F401 is only justified for __init__.py re-exports — move it from the global ignore to [tool.ruff.lint.per-file-ignores] for **/__init__.py so real unused imports elsewhere are flagged.
• F841 has no good reason to be ignored — enable it (remove from ignore).

Acceptance

• F841 enabled, F401 scoped to __init__.py.
• Fix any violations that surface (a handful of unused imports + the dead locals — coordinate with the dead-code removal issue).
• ruff check src/ tests/ clean; CI green.

[johnnichev]

Done in #151 (merged). F401 + F841 are now enforced repo-wide; the F401 ignore is scoped to **/__init__.py. The enforcement is what surfaced the dead code in #137.