Exact ln/exp2 infrastructure for FIR construction (0x1802a24c0 / 0x26b820)

- log2_ln.hpp/cpp: Plugin's exact ln(float) polynomial from 535a70
  (0x1802a24c0). IEEE 754 bit extraction + Horner evaluation.
  Coefficients extracted from binary at 0x181f81f80..0x181f821c0.
  Max error ~3e-6 for typical inputs.

- spectral.cpp: Updated buildFirFromMask to use plugin's ln→negate→exp2
  pipeline instead of naive 1/mask reciprocal.

- exp2_tables.hpp/cpp: Already contains plugin's exp2 tables (0x26b820).

Remaining: twiddle stages (ops B/C/D with cos/sin tables from buf548)
are the missing piece for bit-exact FIR construction. These are
FFT butterflies already implemented in fft.hpp but need integration
into the FIR pipeline.

dsp/log2_ln.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <cstdint>
+#include <cmath>
+#include <cstring>
+
+namespace soothe2 {
+
+// Plugin's exact ln(float) from 0x1802a24c0 (535a70 FFT-conv engine)
+// Computes natural logarithm via mantissa polynomial + exponent scaling
+// Coefficients extracted from binary at 0x181f81f80..0x181f821c0
+// Max error ~3e-6 for typical inputs (x in [1, 1.34))
+float ln_plugin_f32(float x);
+
+// Vectorized version for arrays
+void ln_plugin_f32_arr(const float* in, float* out, size_t n);
+
+}  // namespace soothe2