Drop Python 3.10 once it reaches end of life #174

pyproject.toml declares requires-python = ">=3.10,<4" and [tool.ruff] target-version = "py310",
and the CI matrix in .github/workflows/_checks.yml runs 3.10 through 3.14. Python 3.10 reaches end
of life in October 2026.

Dropping it is not only a floor bump — it is load-bearing for two things this repo has already
written down:

• docs/adr/0007-core-declares-typing-extensions.md records that core carries
  typing-extensions only because pydantic rejects a stdlib typing.TypedDict model on
  Python < 3.12. Raising the floor to 3.10→3.11 does not release it; that ADR's revisit trigger
  fires at 3.12.
• lite_bootstrap/instruments/base.py uses typing_extensions.Self, also 3.11+ in the stdlib.

Work: bump requires-python, target-version, the CI matrix, the install-isolation job's pinned
3.10 interpreter, and the trove classifiers; then re-read ADR-0007 to see whether the floor has
reached 3.12 and the dependency can go.

Revisit trigger: October 2026, or the first upstream dependency that drops 3.10 first.