P2: level-tracker UPDATE-loop located (inline bidirectional IIR in FUN_180529fe0), structural module leveltrack (iir_first_order/bidirectional/unrolled4); scalar==unrolled verified; remaining = live A[] coeffs

dsp/leveltrack.hpp

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+#pragma once
+#include <cstddef>
+
+// Level-tracker UPDATE (structural transcription, NOTES_LEVEL 2026-08-19f).
+// The plugin smooths the per-bin level mask with first-order "leaky-integrator"
+// IIR stages (cumulative running-acc) in FUN_180529fe0. Three stages run in the
+// real per-band loop; the coefficient arrays A[] are runtime values (from the
+// level sidechain / smoothing-param builder). This module provides the exact
+// scalar and vectorized forms as pure functions; callers supply A[].
+namespace leveltrack {
+
+// Scalar first-order stage:  acc=0; y[i]=x[i]*A[i]+acc; acc=y; x[i]=y
+// Mirrors consumers_out.txt scalar loop. In-place on x.
+void iir_first_order(float* x, const double* A, size_t n);
+
+// Bidirectional stage: forward then reverse pass over x with same A.
+void iir_bidirectional(float* x, const double* A, size_t n);
+
+// Vectorized-wide variant handling 4 elements per iteration (like the plugin's
+// unrolled loop). Same math, provided for parity tests.
+void iir_first_order_unrolled4(float* x, const double* A, size_t n);
+
+}  // namespace leveltrack