feat(broadcast): add an SPMC specialization

[tisonkun]

Parent: #206

Goal

Add the asyncband::broadcast::spmc topology when a static single-producer contract provides a clear API or measured implementation benefit over broadcast::mpmc used with one sender.

Current baseline

The unbounded MPMC retention contract is implemented by #117. The bounded MPMC contract remains in #213. An SPMC implementation may be delivered one retention policy at a time; it does not need to wait for the full matrix when the selected policy can be compared with its MPMC counterpart.

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.
• Keep source under asyncband/src/broadcast/spmc; move machinery to crate::internal only when it is genuinely shared outside broadcast.

Acceptance criteria

• Preserve the same public retention contract as the corresponding MPMC variant.
• Compare each specialized path with the matching MPMC broadcast implementation used by one producer.
• Demonstrate that the static sender capability or measured performance benefit justifies a separate public topology.
• Do not retain a separate backend unless its capability or performance benefit justifies the maintenance cost.

The benchmark baseline in #208 is complete. Bounded SPMC work depends on the bounded MPMC contract in #213.