docs: rewrite evidence quality audit as architectural specification

- EvidenceQuality vs EvidenceStrength separation with examples
- Verified is a derived relationship (≥2 independent producers agree),
  not a quality threshold assignable by single tools
- Evidence Gap abstraction: missing → weak → unverified
- Match semantics should eventually replace match-count heuristic
- Fixed 5-level vocabulary (None/Weak/Moderate/Strong/Verified)
- Completion depends on per-Goal minimum quality

Author: bniladridas