feat(broadcast): add a bounded lossless MPMC channel

[tisonkun]

Parent: #206

Status

The unbounded half of the original issue is complete. PR #117 implemented asyncband::broadcast::mpmc::unbounded, and #234 finalized its source layout and public inspection terminology. This issue now tracks only the bounded MPMC broadcast variant.

Goal

Add a bounded MPMC broadcast channel in which every active subscription observes every accepted publication in one committed order and the slowest active subscription applies backpressure.

Contracts

• bounded(capacity) enforces the requested logical capacity and panics if capacity is zero.
• An asynchronous send waits when the slowest active subscription prevents reclamation; try_send reports Full without accepting the value.
• Every accepted publication remains available to every subscription that was active when it was accepted.
• New subscriptions start at the committed tail.
• Multiple producers expose one contiguous committed publication order; later claims cannot become visible before earlier claims.
• Sliding, drop-oldest, overwrite, and lag semantics remain out of scope.

Acceptance criteria

• Cover fanout, concurrent producers, strict capacity, slow-subscription backpressure, reclamation, sender cancellation, receiver cancellation, disconnection, and panic-safe payload cloning.
• Benchmark 1 and 8 producers across representative fanout counts against semantically comparable lossless bounded broadcast channels.
• Compare shared paths with the existing unbounded implementation without forcing both retention policies through an abstraction that obscures either contract.
• Keep ring, cursor, retention, and sequencing types private; put cross-family machinery under crate::internal only when another implementation actually shares it.

The benchmark baseline in #208 is complete. Prior work and discussion: #117, #146, and #167.