Update PR description to reflect unified stream.Message envelope implementation

[Copilot]

The PR description was outdated and referenced the intermediate ordering.Metadata design. The actual implementation uses a unified stream.Message envelope containing both metadata fields and a serialized payload.

Changes

• Updated architecture description: Documents the unified stream.Message proto containing message_seq_no, method, status, entry, and payload fields
• Clarified marshaling approach: Application messages are explicitly serialized to/from the payload bytes field using proto registry-based helpers (unmarshalRequest, unmarshalResponse)
• Documented package rename: ordering → stream to better reflect the envelope's purpose
• Added encoding layer details: Describes the Message wrapper struct and helper functions (NewResponseMessage, AsProto, messageWithError)
• Specified error handling improvements: Server marshaling failures now send error responses instead of closing the stream

The implementation eliminates the custom codec by leveraging gRPC's type-safe Send/Recv with explicit payload serialization.

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[deepsource-io]

Here's the code health analysis summary for commits 23f3e46..98619af. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Go	✅ Success		View Check ↗
Shell	✅ Success		View Check ↗

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[meling]

This is annoying... I didn't ask for a new PR.