build(channel): consolidate feature flags and documentation

[tisonkun]

Parent: #206

Goal

Stabilize Cargo feature grouping and public documentation after the channel families that will ship in 0.7 are known.

Direction

• Keep oneshot available as a small standalone capability.
• Evaluate one channel umbrella for the implemented multi-message channel families.
• Do not expose private storage or queue taxonomy as Cargo features.
• Keep public modules at their direct crate-root paths.

Acceptance criteria

• Measure compile-time and metadata impact before retaining fine-grained topology features.
• Minimize feature-matrix and conditional-documentation complexity.
• Document exactly which public modules each retained feature enables.
• Make any feature removals before the 0.7 public API is released.

This is a finalization task and should follow the implementation decisions in the other sub-issues.