Add a Python 3.11+ resolving launcher

[mlava]

Summary

• add an executable ./ringer shim that probes Python 3.13 → 3.12 → 3.11 → python3
• verify the candidate is actually Python 3.11+ before executing ringer.py
• preserve caller arguments and support arbitrary working directories and paths with spaces
• recommend the launcher in Quickstart while retaining direct interpreter invocation

Verification

• focused fake-PATH launcher tests
• python3 -m unittest discover -s tests -v (222 tests)
• independent Opus diff review: APPROVE

Closes #44

[mlava]

CI note: Ubuntu passes, and the full suite passed locally before this PR was opened. macOS has failed twice in unrelated pre-existing signal-shutdown timing tests (test_sigterm_cleans_up_active_worker_and_finishes_state, then test_second_signal_during_shutdown_does_not_cancel_cleanup), while the launcher tests themselves pass. I pushed a retry commit because fork PR authors cannot rerun upstream Actions, but the retry hit the sibling timing race. This appears unrelated to the launcher diff; both failures show the temporary run-state replace racing shutdown.