feat(v2.4.1): add OutputAssertion for non-zero and varies-from output checks

[Copilot]

Summary

Adds a new OutputAssertion enum to SnapshotConfig that applies a correctness assertion to output samples after audio unit processing.

New API

pub enum OutputAssertion {
    /// Assert that at least one output sample (across all channels) is not 0.0. (default)
    NonZero,
    /// Skip the output assertion entirely.
    Skip,
    /// Assert that at least one output sample differs from the given baseline value.
    VariesFrom(f32),
}

The field is set via the builder:

// Default — panics if all output is silence
let config = SnapshotConfigBuilder::default().build().unwrap();

// Opt out (e.g. for filter units driven by InputSource::None)
let config = SnapshotConfigBuilder::default()
    .output_assertion(OutputAssertion::Skip)
    .build().unwrap();

// Check output varies from an arbitrary baseline
let config = SnapshotConfigBuilder::default()
    .output_assertion(OutputAssertion::VariesFrom(0.5))
    .build().unwrap();

Changes

• src/config.rs — new OutputAssertion enum + output_assertion field on SnapshotConfig / SnapshotConfigBuilder (default: NonZero)
• src/snapshot.rs — applies the assertion after all samples are collected in capture_audio_unit_data
• src/tests.rs — two existing tests that use silent filters opted out via Skip; five new tests cover all variants and panic cases
• Cargo.toml — version bumped to 2.4.1