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.cpp

@@ -0,0 +1,48 @@
+#include "log2_ln.hpp"
+#include <cstring>
+#include <cmath>
+
+namespace soothe2 {
+
+float ln_plugin_f32(float x) {
+    if (x <= 0.0f) return -INFINITY;
+    
+    uint32_t bits;
+    std::memcpy(&bits, &x, sizeof(uint32_t));
+    int exp = int((bits >> 23) & 0xFF);
+    uint32_t mantissa = bits & 0x7FFFFFu;
+    
+    if (exp == 0) return -INFINITY;
+    
+    float x_norm = mantissa * (1.0f / 8388608.0f);
+    
+    constexpr float c0_a = -0.1517720520f;
+    constexpr float c0_b =  0.1696488112f;
+    constexpr float c1   = -0.1646245718f;
+    constexpr float c2   =  0.1982250363f;
+    constexpr float c3   = -0.2500466406f;
+    constexpr float c4   =  0.3333656490f;
+    constexpr float c5   = -0.5000000000f;
+    constexpr float ln2  =  0.6931471825f;
+    
+    constexpr float c0_init = c0_a * c0_b;
+    
+    float y = c0_init + x_norm;
+    y = y * x_norm + c1;
+    y = y * x_norm + c2;
+    y = y * x_norm + c3;
+    y = y * x_norm + c4;
+    y = y * x_norm + c5;
+    
+    float ln_m = x_norm + x_norm * x_norm * y;
+    
+    return ln2 * float(exp - 127) + ln_m;
+}
+
+void ln_plugin_f32_arr(const float* in, float* out, size_t n) {
+    for (size_t i = 0; i < n; ++i) {
+        out[i] = ln_plugin_f32(in[i]);
+    }
+}
+
+}  // namespace soothe2