bug: serialize send to allow for concurrent session use

[nemith]

While doing some performance testing it was clear that I ditched the previous serialization for writes. The old v0.0.2 method was just to put all sends behind a mutex. This is a bit more advanced using an outbound queue and writing each message as they come in.

[Copilot]

Pull request overview

This PR introduces serialized message sending to enable concurrent session use. Previously, the code allowed concurrent writes which could cause issues with the transport layer that only supports one active writer at a time. The new implementation uses an outbound message queue processed by a dedicated goroutine to serialize all writes.

Key changes:

• Added sendLoop() goroutine and outQ channel to serialize all outbound messages
• Renamed pendingReq to pendingResp for better semantic clarity
• Refactored Do() method to use queueSend() instead of writing directly to transport

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
session.go	Implements outbound message queue with sendLoop(), queueSend(), and sendMsg() functions; adds outQ channel to Session struct; refactors Do() to use queue-based sending; starts sendLoop goroutine after handshake
session_test.go	Updates test to use renamed pendingResp type instead of pendingReq

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

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