refactor(channel): establish private source layout

[tisonkun]

Closes #207.

Summary

• Collect the existing oneshot and MPSC implementations under one private channel source module while preserving the direct asyncband::oneshot and asyncband::mpsc public paths.
• Keep the existing MPSC error module local to MPSC instead of introducing a shared channel error boundary before another implementation needs it.
• Keep algorithms, behavior, Cargo features, dependencies, tests, and documentation contracts unchanged.
• Validate the full feature matrix, workspace tests, lint and docs, and semver compatibility against the pre-change main revision.