Fix lifetime invariance in FrameIterator and WindowIterator

[Copilot]

AudioSamples<'a, T> is invariant in 'a, so using a single lifetime for both the reference-to-AudioSamples and its internal data lifetime made .frames() and .windows() unusable from any function taking &AudioSamples<T> — the compiler required the caller to explicitly write &'a AudioSamples<'a, T> with unified lifetimes.

// Previously failed to compile:
fn windowed_mean(audio: &AudioSamples<f32>) -> Vec<f32> {
    audio.windows(nzu!(1024), nzu!(512))
        .map(|w| { /* ... */ })
        .collect()
}
// error: lifetime may not live long enough
//  = note: the struct `AudioSamples<'a, T>` is invariant over the parameter `'a`

Changes

• FrameIterator<'a, T> → FrameIterator<'s, 'a, T> — separates the reference lifetime 's from the data lifetime 'a, with an 'a: 's bound. Matches the pattern ChannelIterator already used correctly.
• WindowIterator<'a, T> → WindowIterator<'s, 'a, T> — same treatment.
• AudioSamples::frames() / ::windows() — changed from &'a self to &'s self, decoupling the borrow lifetime from the data lifetime.
• AudioSampleIterators trait — updated frames and windows signatures to match.
• next() implementations — explicitly copies the stored &'s reference before calling slice_samples to preserve the full 's lifetime through the method call.
• Regression tests — added test_frames_on_borrow, test_channels_on_borrow, test_windows_on_borrow, and test_frames_and_windows_same_borrowed_ref to guard against recurrence.

Warning

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• googlechromelabs.github.io
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