Support Python 3.10 and align typing/CI tooling

[Code0x58]

Motivation

• Broaden runtime and packaging support to Python 3.10 while keeping the public decorator API type-checkable.
• Make local development and CI use the same project-managed uv environment instead of relying on separately installed tools.
• Tighten contributor and CI checks so formatting, linting, typing, tests, and 100% line coverage stay explicit.

Description

• Update package metadata for Python >=3.10, add Python 3.10/3.11 classifiers, and expand the CI matrix to run on Python 3.10 through 3.14.
• Refactor alternative.py annotations to Python 3.10-compatible Generic, ParamSpec, and TypeVar forms, replacing newer type-parameter syntax and removing the previous pyrefly overload ignore.
• Rework the reference, Alternatives.add, Implementation.add, and pytest helper decorator overloads around a typed _Undefined sentinel so bare-decorator and configured-decorator usage are represented directly.
• Add ruff, pyrefly, and mypy as project dev dependencies and simplify CI to rely on uv sync --dev plus uv run --dev ... commands.
• Enforce 100% line coverage in CI with --cov-fail-under=100, document the full verification command set in AGENTS.md, and require branch-coverage review for material alternative.py branching changes.
• Ignore generated test-results.xml alongside the other coverage/test artifacts.

Testing

• uv run --dev ruff format --check .
• uv run --dev ruff check .
• uv run --dev pyrefly check .
• uv run --dev mypy .
• uv run --dev pytest
• uv run --dev pytest --cov-report=term-missing:skip-covered
• uv run --dev pytest --cov-branch --cov-report=term-missing:skip-covered
• uv run --dev pytest -vv --cov=alternative --cov-report=xml --cov-fail-under=100 --junitxml=test-results.xml

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (25629aa) to head (4af57a9).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          196       198    +2     
=========================================
+ Hits           196       198    +2     

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