This change adds full return-type and parameter-type annotations to all
~625 functions and methods across the entire virtualenv source tree. It
brings the codebase from ~26% type coverage to near-complete coverage,
matching the standard already established in the sibling tox project.

Key changes include:

- PEP 561 marker: Added `py.typed` so downstream consumers get inline
type information.
- Ruff ANN rules enabled: Replaced the blanket "ANN" ignore in
`pyproject.toml` with only the two specific exceptions (ANN003, ANN401),
and added FBT001 to the global ignore since the existing
boolean-positional-argument function signatures are public API.
- ty type checker hardened: Added --error-on-warning to both `type` and
`type-3.8` tox environments in `tox.toml`.
- All modules annotated: config, run, create (including
CPython/PyPy/GraalPy/RustPython builders), activation (all 7 shells),
seed (pip install, wheels, periodic update), app_data, util (lock, path,
subprocess, zipapp), and discovery — every function now has explicit
parameter types and return types.
- Both linters pass clean: ruff check and ty check --python-version 3.14
--error-on-warning report zero errors. All pre-existing tests continue
to pass.

Fixes: #3075 

### Thanks for contributing, make sure you address all the checklists
(for details on how see [development
documentation](https://virtualenv.pypa.io/en/latest/development.html#development))

- [x] ran the linter to address style issues (`tox -e fix`)
- [x] wrote descriptive pull request text
- [x] ensured there are test(s) validating the fix
- [x] added news fragment in `docs/changelog` folder
- [x] updated/extended the documentation

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>