Add equilibrium-padded interpolator constructors

[orange-dot]

Closes #121.

Summary

Adds equilibrium_padded constructors for Floor, Linear, and Sinc interpolators.

Floor and Linear initialize their interpolation history with Frame::EQUILIBRIUM. Sinc clears the supplied ring buffer to Frame::EQUILIBRIUM before use, preserving the existing Sinc::new behavior for callers that intentionally provide custom padding.

Validation

Ran formatting and workspace tests locally, including dasp_interpolate, dasp_signal, and cargo test --locked --workspace --all-targets.

Sonic smoke

A local startup-transient A/B render confirmed the behavior: dirty Sinc::new had peak 1.10 in the first 128 samples, while Sinc::equilibrium_padded had peak 0.0 and matched the zero-buffer baseline with max abs diff 0.0.