Event schema documentation + on-chain/off-chain parity test

[joelpeace48-cell]

Epic: Contract Safety & Hardening

Summary

Document every emitted event and assert the indexer parses each exactly.

Background & Problem

Event/indexer drift silently corrupts derived state; a parity test prevents it.

Acceptance Criteria

• Every event documented.
• Indexer-parse parity test passes.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.