The invariant precedence chain is four-tier per specification §92:
plan > action > project > global
This fix updates:
1. Module docstring in invariant.py to document the correct four-tier precedence
2. InvariantScope class docstring to reflect PLAN > ACTION > PROJECT > GLOBAL
3. merge_invariants() function to accept action_invariants parameter
4. InvariantSet.merge() class method to accept and pass action_invariants
5. InvariantService.get_effective_invariants() to collect and pass action invariants
6. BDD test steps to include action invariants in merge operations
7. Benchmark suite to include action invariants in performance tests
8. Robot Framework helper to pass action_invariants to merge functions
9. CHANGELOG.md entry under [Unreleased]/### Fixed section
10. CONTRIBUTORS.md entry documenting HAL 9000 contribution
All docstrings now correctly document the four-tier precedence chain,
and the merge logic properly handles action-scope invariants between
plan and project scopes.
ISSUES CLOSED: #9003
HAL9000