Ivan FB b970e49570
refactor(protobuf): proto3 optional (Opt) singular fields + app-layer validation
Satisfies the review feedback in one coherent design:
- proto3 (no proto2 `required`, which is discouraged and only checks presence)
- singular fields use the proto3 `optional` label (Opt[T]) — recommended over
  implicit fields for forward-compatibility
- opaque seq[byte] ids (no UTF-8 validation)
- mandatory identifiers (messageId/channelId and each entry's messageId) are
  validated at the application layer after decoding

Opt-in-proto3 requires protobuf_serialization >= 0.5.0 (already pinned to 0.5.1).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
2026-06-11 16:19:21 +02:00
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