v0.12.4

Runtime cleanliness refactor release.

• narrows RuntimeEngine into a stable facade backed by dedicated lifecycle and tick-cycle collaborators
• splits routed post-stage mutation into dedicated runtime modules for counters, work-item transitions, handoff incidents, persistence, and closure-target handling
• tightens runtime characterization coverage and updates runtime architecture documentation

Verification:

• uv run --extra dev python -m pytest -q
• uv run --with ruff ruff check src/millrace_ai tests
• uv run --with mypy mypy src/millrace_ai