Module backpressure 

Backpressure: BytePermits

Byte-based flow control for write pumps.

Design principle:

• Backpressure scales with bytes, not message count
• One giant message should not starve other connections
• Pluggable: NoOp (default) → Semaphore → dynamic policy

Usage:

let permits = SemaphorePermits::new(10 * 1024 * 1024); // 10MB limit
let permit = permits.acquire(n_bytes).await;
writer.write(buf).await;
drop(permit); // releases automatically

Structs

NoOpPermits

No-op implementation (Phase 0).

Permit

RAII permit guard.

SemaphorePermits

Semaphore-based backpressure implementation.

Traits

BytePermits

Backpressure permit trait.