What's the problem this feature will solve?

virtualenv currently has very limited type annotation coverage (~26%). The sibling project tox already ships with ~92% coverage and a py.typed marker. This means:

• Downstream consumers and plugins don't get type checking benefits when importing from virtualenv
• IDE autocompletion and inline documentation are degraded for the public API (cli_run, session_via_cli, etc.)
• Contributors have no type-level guardrails catching bugs at development time
• The existing ty type checker environments in tox.toml can't enforce correctness without annotations to check

Describe the solution you'd like

• Add return-type and parameter-type annotations to all functions and methods across the source tree
• Add a PEP 561 py.typed marker file so type checkers recognize virtualenv as an inline-typed package
• Enable ruff's ANN lint rules (currently blanket-ignored with the comment "no type checking added yet"), keeping only ANN003 and ANN401 as targeted exceptions
• Harden the ty check tox environments with --error-on-warning to prevent regressions

Alternative Solutions

• Maintain a separate virtualenv-stubs package — higher maintenance burden, easily falls out of sync
• Use stub files (.pyi) alongside source — duplicates every signature, same sync problem
• Inline annotations (proposed) match how tox already handles this and are the lowest-maintenance path

Additional context