Adopt XDR Views: View-based event filtering + response

[cjonas9]

Summary

The event scan loop Unmarshals on every row regardless of whether or not the scanner callback filters the events. This decoding work can be avoided.

Scope

Files: cmd/stellar-rpc/internal/db/event.go, cmd/stellar-rpc/internal/methods/get_events.go

• db/event.go:428 event scan loop: event.UnmarshalBinary(row.eventData) per row.
  • We should wrap row.eventData as DiagnosticEventView and run filter checks against view accessors (contract ID, type, topic patterns)
  • Only call event.UnmarshalBinary when filter passes AND the scanner callback genuinely needs the decoded form
• get_events.go:296: xdr.MarshalBase64(segment) for each topic in non-JSON response output + get_events.go:304: xdr.MarshalBase64(v0.Data) for event data in non-JSON response output
  • We should slice topic and data bytes from the source via view rather than re-encoding
• Possibly change ScanFunction signature to accept view (or both).

Acceptance criteria

The decoding work done in getEvents query handling no longer decodes in filtering and no longer encodes its responses.