Split ChannelManager::read into two stages

Introduce ChannelManagerData<SP> as an intermediate DTO that holds all
deserialized data from a ChannelManager before validation. This splits
the read implementation into:

1. Stage 1: Pure deserialization into ChannelManagerData
2. Stage 2: Validation and reconstruction using the DTO

The existing validation and reconstruction logic remains unchanged;
only the deserialization portion was extracted into the DTO's
ReadableArgs implementation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: joostjager