fix: enforce unique receiver identities in signature verification

[zsculac]

Summary

• Adds a unique identity count check in _verifySignatures() for KnowledgeCollection
• Prevents a single node from faking multi-party attestation by submitting duplicate identity IDs that pass individual signature verification
• Reverts with "Insufficient unique receiver identities" only when the number of unique identities falls below minimumRequiredSignatures — duplicates are tolerated if enough unique identities remain

Test plan

• New test: all-duplicate receiver identities revert
• All 6 KnowledgeCollection unit tests pass

🤖 Generated with Claude Code

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Note

High Risk
Touches signature-threshold enforcement in a Solidity contract; mistakes here can allow forged multi-party attestation or inadvertently reject valid submissions.

Overview
Prevents meeting minimumRequiredSignatures via duplicated receiver identityIds by counting unique identities in KnowledgeCollection._verifySignatures and reverting when the unique count is below the threshold.

Adds unit coverage for the new behavior: all-duplicate receiver lists now revert with "Insufficient unique receiver identities", while mixed lists with enough unique identities still succeed.

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