refactor(channel): establish an intermediate private source layout

[tisonkun]

Follow-up

This issue records the intermediate private source layout that was implemented before 0.7. PR #234 subsequently removed the private channel wrapper and placed oneshot, mpsc, broadcast, and watch directly under asyncband/src/, matching their public crate-root modules. The public paths and feature flags remained unchanged. Shared cross-family machinery now belongs under the private crate-level internal module.

Parent: #206

Goal

Move the existing oneshot and MPSC implementations into the terminal private channel source family without changing their public API, behavior, feature flags, or algorithms.

Scope

• Add a private channel module below the crate root.
• Move the existing oneshot and mpsc source trees under asyncband/src/channel/.
• Keep MPSC-specific error definitions inside the mpsc implementation until another channel family demonstrates a useful shared contract.
• Re-export oneshot and mpsc at the crate root so all existing public paths remain unchanged.
• Update internal visibility and documentation links required by the move.

Non-goals

• No MPSC algorithm rewrite.
• No new channel family or constructor.
• No channel feature consolidation.
• No public asyncband::channel module.

Acceptance criteria

• Existing asyncband::oneshot and asyncband::mpsc APIs remain source-compatible.
• Builds with no features, oneshot, mpsc, and all features succeed.
• The workspace test and lint workflows pass, and a semver comparison against the pre-change main revision reports no API changes.
• The pull request remains a move-only structural change apart from visibility and path adjustments required by the new ownership boundary.