feat(broadcast): add lossless MPMC channels

[tisonkun]

Parent: #206

Goal

Add bounded and unbounded MPMC broadcast channels in which every active subscription observes every accepted publication in one committed order.

Contracts

• Bounded broadcast applies backpressure at a strict logical capacity and is gated by the slowest active subscription.
• Unbounded broadcast grows subject to memory and reclaims values after every active subscription advances or drops.
• New subscriptions start at the committed tail.
• Sliding, drop-oldest, and lag semantics remain out of scope.

Acceptance criteria

• Cover fanout, concurrent producers, contiguous publication order, slow subscriptions, reclamation, cancellation, disconnection, and panic-safe payload cloning.
• Benchmark 1 and 8 producers across representative fanout counts against lossless broadcast peers.
• Keep ring, cursor, retention, and sequencing types private.

Depends on #208. Prior work and discussion: #117, #146, and #167.