channel: keep actors alive across disconnects

[jjyr]

Purpose

The Network actor is becoming single point of the system.

This PR keeps the channel actor alive even peer is disconnected, so we can offload channel's bussiness logic from network actor to channel actors.

Summary

• Keep channel actors alive across peer disconnects instead of stopping and recreating them.
• Introduce explicit channel connectivity handling (Offline -> Syncing -> Online) and let the channel actor drive reestablish on reconnect.
• Add official ChannelOnline / ChannelOffline service events so reconnect tests can wait on channel connectivity instead of actor recreation or fixed sleeps.
• Offload channel tlcs timeout check from network actor to channel actors.

Secondary changes

• Rework reconnect bookkeeping around Pubkey so requested disconnects and backoff reconnects stay aligned with long-lived actors.
• Preserve retryable remove-TLC preimages during reestablish and avoid settling forwarding TLCs that are still waiting on downstream results.

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Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 5 comments.