[bdbarnett]

Ladder.set(cutoff_hz=...) moves the cutoff once a block. That's fine for an envelope or a knob, but sweep the cutoff with an audio-rate signal that way and you get a staircase, which you hear as a buzz at the block rate.

This adds a cutoff audio input, following the way audiooscillator.Oscillator takes its pitch:

ladder = audioladder.Ladder(cutoff_hz=800.0, resonance=3.6, cutoff_octaves=2.0)
ladder.modulate(cutoff=lfo)   # or Ladder(..., cutoff=lfo)

The cutoff becomes cutoff_hz * 2 ** (v * cutoff_octaves) for an input at v of full scale. Only the first channel is read, and the result is clamped to the same band cutoff_hz is. cutoff_octaves defaults to 10, which is the oscillator's pitch scale. An input of zero, an input that has run dry, or no input at all gives you the configured filter bit for bit.

The per-sample coefficients use a deterministic exp2 (moved out of the oscillator into shared/audiodsp_exp2.h) and a sin/(sin + cos) series instead of libm's tanf, so every target renders them the same. A held input value is cached. The unmodulated path's output is unchanged: the ladder probe gives identical bytes before and after. Its speed is unchanged too, at 23.7-24.1 µs per stereo 256-frame block both ways. The modulated path costs about 15% more.

The MicroPython binding, the CircuitPython spike and CPython all get the same surface: modulate(), the cutoff keyword and cutoff_octaves.

Tests, in tests/test_cpython_audioladder_cutoff.py:

• A zero input renders the same bytes as no input.
• A constant input matches set() at the cutoff it names, within 1 LSB.
• Audio-rate FM with a 1900 Hz carrier, a 100 Hz modulator and 64-frame blocks at 48 kHz. With the input, the 750 Hz block-rate spur is 190 dB under the carrier. Stepped once a block with set(), the control, it's 18 dB under.
• A dry input counts as zero, a stereo input is read from its first channel, and modulate() refuses names and values that aren't inputs.

ladder_probe.py gains swept, held and stereo-input fixtures, so verify_dsp compares the new path across interpreters. Locally, MicroPython v1.29.0 unix and CPython agree on all 58 comparisons.