Clarify verifier causes, evidence gaps, and preview routing

[pengfei-threemoonslab]

Summary

• name the deterministic worst blocker in ordinary blocked verifier headlines and propagated control prose
• distinguish fully enumerated lower-confidence surfaces from genuinely incomplete enumeration
• consume embedded trigger advice so verifier control is the only operational next action
• replace engine-owned policy IDs with adopter-facing mixed-evidence guidance while preserving public check labels
• document the projection changes and update generated contracts and sample goldens

Testing

• PYTHONPATH=src python -m pytest -q --ignore=tests/test_packaging.py
• PYTHONPATH=src pytest tests/test_packaging.py -q
• python -m ruff check .
• git diff --check
• ref-bound Agents Shipgate verify: complete / passed / mergeable

Closes #436
Closes #396
Closes #414
Closes #420