[arike-tech]

Closes #10

Summary

• Add parse_contract_event_xdr to decode base64 ContractEvent blobs with stellar-xdr.
• Normalize contract id, event type, topics, and data into the existing EventRecord shape.
• Accept raw blobs directly through parse_events_str and as xdr/eventXdr fixture fields.
• Preserve optional fixture metadata such as ledger and txHash when supplied alongside a raw event.
• Return structured decode errors for malformed base64/XDR.
• Update docs/events.md and the changelog.

Tests

• cargo test -p observatory-events passed.
• cargo test --workspace --all-features passed.
• cargo clippy -p observatory-events --all-targets --all-features -- -D warnings passed.
• Added tests for raw event decoding, fixture-entry decoding, bare blob input, malformed input, and fixture-shape equivalence.