fix(buffer): assert non-zero bound on construction

[Alpy16]

Closes #861

Description
Initializing Buffer or BufferLayer with a capacity bound of 0 creates an invalid configuration state. This bypasses initial validation and causes an unrecoverable runtime panic down the line when the underlying mpsc::channel allocation is executed.

This patch adds a defensive assert!(bound > 0, ...) boundary guard to both the BufferLayer::new and Buffer::pair constructors. This shifts a latent runtime crash into a predictable, fail-fast initialization panic.

Type of Change
Bug fix (non-breaking change enforcing defensive API boundaries)

Verification
Added #[should_panic] integration tests to tower/tests/buffer/main.rs to verify that both direct instantiation and layer application fail safely when configured with a bound of 0.

Verified workspace tests pass via cargo test --test buffer --all-features.

[seanmonstar]

Probably the docs on the constructor should also mention that the value must be greater than 0, maybe in a # Panics section?

[Alpy16]

I'll add the # Panics section to the documentation for both constructors right now and get those pushed up. Thanks for the review