feat(broadcast): add an SPMC specialization

[tisonkun]

Parent: #206

Goal

Add the asyncband::broadcast::spmc topology after the lossless broadcast contracts and shared retention machinery are established.

Direction

• Expose one non-cloneable sender with exclusive send methods.
• Keep subscriptions independent and explicitly created.
• Evaluate whether a single-producer sequencer or Disruptor-style bounded ring provides a measured benefit; keep it private either way.

Acceptance criteria

• Preserve the same bounded and unbounded retention contracts as MPMC broadcast.
• Compare the specialized path with MPMC broadcast used by one producer.
• Do not retain a separate backend unless its capability or performance benefit justifies the maintenance cost.

Depends on #213 and #208.