Adds 2 invariant tests:
- test_phase_8_invariant_property_setter_count_dropped: pins the count
to exactly 0 (post-Phase-8 baseline; all 22 INTERNAL_BROAD_CATCH sites
in src/gui_2.py migrated across Phases 3-8).
- test_phase_8_invariant_all_2_migration_sites_have_tests: verifies the
2 migrated sites (L591, L897) have both success and failure tests.
Updates state.toml: phase_8 status = completed.
ed
7ec512c792