feat!: drop Python 3.10 and use typing.assert_never

[lesnik512]

Closes #33.

Python 3.10 reaches end of life on 2026-10-31. This drops it ahead of that date rather than waiting for the trigger recorded in ADR-0002.

Changes

• requires-python is now >=3.11,<4, and the 3.10 classifier is gone. Ruff takes its target from requires-python, so no [tool.ruff] target-version is added.
• CI: lint now runs on 3.11, and "3.10" is dropped from both the pytest and floors matrices. The issue only mentioned pytest.
• fixer.py: the vendored _assert_never is replaced with typing.assert_never. The exhaustive match keeps its case _: arm.
• ADR-0002 ("keep 3.10, vendor _assert_never") is deleted, not superseded. Its decision no longer holds, and using the stdlib helper on 3.11+ does not meet the FL4 bar for a record of its own. The ADR tree in docs/agents/domain.md is updated; it was also missing 0003.

The issue cites "ADR-0004", but the record is 0002.

Verification

• On 3.11: just lint-ci, just test-ci (100% coverage) and just adr-check all pass.
• With the Truncate arm temporarily removed, ty check reports type-assertion-failure at assert_never(action), so the exhaustiveness guard still works.

Release

This is a breaking change because it drops a supported interpreter. Release it as a minor or major version, not a patch.