docs: make counter-evidence first-class

[cschanhniem]

Summary

Adds a counter-evidence ledger standard for Open Problem Lab.

This is deliberately not a new validator yet. It is a governance/documentation PR that defines how failed replications, null results, external-validity limits, dataset flaws, field contradictions, and unresolved reviewer objections should be preserved instead of disappearing into closed issues.

Why this matters

The highest-risk failure mode for a verification protocol is not lack of impressive accepted claims. It is selection bias: accepted evidence accumulates while disconfirming evidence becomes hard to find.

The new document introduces:

• what belongs in a counter-evidence ledger;
• a lightweight entry format;
• status semantics for unresolved/bounded/resolved/withdrawn claims;
• reviewer and agent rules;
• anti-patterns to avoid debate-dump behavior;
• an acceptance test centered on the strongest known falsifier.

Safety / scope

Docs-only. No schema, workflow, or canonical problem-pack content is changed.

Review focus

Please review whether this should become a required artifact for mature problem packs, or stay advisory until a pack has enough accepted claims to justify it.