fix: harden effect verifier candidate preflight

[abrichr]

Summary

• keep different-path on-screen reacquisition eligible when its pre-state is intentionally unreadable
• make candidate and redaction-wrapper connection probes safe and non-throwing
• reject incomplete verifier plugins at construction before a run reaches delivery

Safety

The selected candidate remains pinned before actuation. A non-confirmed or indeterminate proof never becomes VERIFIED, and the runtime does not downgrade after delivery.

Validation

• uv run pytest -q tests/test_effect_verifier_candidates.py tests/test_effect_kit_config.py tests/test_verifier_adapter_platform.py tests/test_onscreen_verifier.py
• uv run pytest -q tests/test_replayer_api_actuator.py tests/test_replayer.py
• Ruff check and format check
• mypy plus strict safety-path type check
• uv run python scripts/check_release_consistency.py

[abrichr]

Superseded by #323. Its head includes every commit from b9664f0 and adds the required live-backend binding for selected on-screen candidate verifiers. Keep #323 as the single effect-verifier candidate PR.