Add OnePoleBank: phasor-free resonator core (~1.4–1.7× faster)

[enomado]

What

Adds OnePoleBank, a reformulation of the resonator core that drops the per-sample running phasor and the periodic stabilization it requires, while producing bit-identical power/magnitude. It's ~1.35–1.70× faster than the current ResonatorBank on x86-64-v3 (AVX2), and is numerically self-stabilizing.

It's added alongside ResonatorBank — purely additive, no behavior change to existing code. Happy to fold it into ResonatorBank or make it the default core if you like the approach.

Why it works

The current resonator keeps, per bin, a unit phasor z rotated by e^{-iωΔt} every sample, accumulates α·x·z into the EWMA, and must periodically renormalize z because |w| != 1 exactly in f32 (so z drifts) — hence STABILIZE_EVERY and the sqrt renorm.

A resonator is a heterodyne followed by a one-pole lowpass. Move into the de-rotated frame u_n = r_n · e^{+iωnΔt} and the "rotate phasor + EWMA" pair collapses into a single complex one-pole with a constant complex coefficient:

u_n = c·u_{n-1} + α·x_n      c = (1-α)·e^{+iωΔt}
v_n = d·v_{n-1} + β·u_n      d = (1-β)·e^{+iωΔt}   (the output-smoothing EWMA)

Two facts make this a strict win:

1. Power/magnitude are unchanged. |e^{-iωnΔt}| = 1, so |v_n| ≡ |rr_n| exactly. For spectrogram/magnitude use the phasor was never needed.
2. It's self-stabilizing. |c| = (1-α) < 1 strictly, so the recurrence is contractive — round-off decays instead of accumulating. There is nothing to renormalize. STABILIZE_EVERY, the sqrt, and the chunk-around-stabilize loop in process_samples all go away.

The absolute-frame complex value (and phase) is still available — recovered by de-rotating v by e^{-iωnΔt} at readout only (hop boundaries), instead of maintaining a phasor every sample. The de-rotation argument grows without bound, so it's reduced in f64 (see Correctness).

Correctness (all in this PR)

Power/magnitude is algebraically equal to ResonatorBank for any (freq, α, β) — only f32 round-off separates them — so the bar is parity, not "close enough":

• Fixture parity — tests/reference_onepole.rs runs against the same chirp_88.npz fixture as reference.rs:
  • power matches to 9.9e-5 relative (pure f32 round-off — the two are mathematically identical),
  • complex matches to 6.1e-4 via lazy de-rotation.
• Property-based parity — a proptest fuzzes the consumer's operating point: independent alpha_scale/beta_scale (so α != β), the full MIDI 12..=84 range including the high-Q low end where heuristic_alpha is tiny, and an arbitrary multi-partial stimulus (96 cases), asserting OnePoleBank powers track ResonatorBank within 0.5 % rel + 0.1 % peak. Worst case measured on a real 5-bins/semitone grid: ~6e-5 relative (~1e-4 with extreme α != β) — far below any display threshold, so a ResonatorBank → OnePoleBank swap cannot change a magnitude/power readout.
• Long-run phase stability — phase()/complex() de-rotate v by e^{-iωnΔt}, whose argument grows without bound; it's formed and reduced in f64 so a long-running readout stays accurate (at f32 the trig argument degrades within minutes). phase_readout_stable_at_large_sample_count recovers a +1 Hz detuning from two phase samples 128 apart after ~4 M samples — which fails outright if the de-rotation is done in f32.
• Inline test asserts power equality vs ResonatorBank on a multi-tone signal.

Full crate suite green: 27 unit (incl. the property test) + 2 + 2 fixture/integration + 2 doc.

SIMD preserved

Same SoA layout and the same mul_add (with the existing wasm32+simd128 unfuse). Confirmed in the release asm that the hot loop autovectorizes to AVX2 vfmadd231ps over ymm (8 resonators/iteration) reading the strided SoA arrays — i.e. it vectorizes the same way ResonatorBank does.

Benchmark

benches/compare.rs benches both banks side by side (1 s signal, per-sample). On x86-64-v3 / AVX2:

bins	phasor (current)	onepole	speedup
88	29.0 M/s	49.2 M/s	1.70×
264	12.0 M/s	17.5 M/s	1.46×
440	7.4 M/s	10.6 M/s	1.44×
880	3.6 M/s	4.8 M/s	1.35×

The speedup is largest when the working set is cache-resident (compute-bound, small bin counts) and shrinks as it becomes memory-bandwidth-bound — consistent with the change being a per-sample arithmetic reduction (the phasor rotate is gone).

Files

• src/bank_onepole.rs — OnePoleBank (full read API + inline + property-based parity tests)
• src/lib.rs — export
• tests/reference_onepole.rs — fixture parity (power + complex)
• benches/compare.rs + Cargo.toml — side-by-side bench
• Cargo.toml — proptest dev-dependency (property-based parity)

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