design(spsc): decide whether to expose a dedicated queue

[tisonkun]

Parent: #206

Goal

Determine whether a distinct public SPSC queue provides enough static API value or measured implementation benefit to justify a dedicated module.

Questions

• Is a non-cloneable sender and receiver contract materially easier to use correctly than MPSC with one sender?
• Can a specialized backend improve latency, allocation, or synchronization without creating disproportionate maintenance cost?
• Should SPSC initially be a nominal wrapper over the single-consumer core or wait for a specialized backend?

Acceptance criteria

• Compare the proposed API and implementation with using MPSC directly.
• Benchmark bounded and unbounded SPSC against the MPSC one-producer path and relevant ecosystem channels.
• Record an explicit implement-or-defer decision. If implemented, preserve non-cloneable endpoints and exclusive &mut self operations without exposing backend markers.

Depends on #208 and #209.