OIC near-duplicate claim and contradiction detection #238

Context

v1 (#227) catches only exact duplicates and obvious contradictions. Start after #235.

Behavior to implement

• Detect near-duplicate claims, including across entity aliases, and propose a merge instead of creating a second claim.
• Detect likely contradictions between claims with overlapping scope and valid time.
• Prefer deterministic matching (normalized subject, predicate, and object, plus aliases) before anything model-based.

Acceptance criteria

• Two proposals saying the same thing through different entity aliases end up as one claim with both pieces of evidence.