spectral: vectors instead of new[]; vlaw: extract law + vlaw_check target

- spectral.cpp: window_/buf_/tmp_buf_/fir_buf_/fir_freq_ as std::vector (no
  exception-leak in ctor, destructor = default)
- framed_model.hpp: extract vlaw_cut/vlaw_mask inline (BLOCKMAP:314 softplus)
- dsp/vlaw_check.cpp: unit test for law (monotonic, zero-level, delta, ref,
  comb-neutral) — PASS
- CMake: add vlaw_check target
- Guard: corpus --compare d=+0.000, fn529fe0_check PASS, twin_check PASS

dsp/framed_model.cpp

@@ -316,10 +316,9 @@ static void process_band_structural(
         if (delta_state) {
         }
         for (size_t k2 = 0; k2 < nbin; k2++) {
-            double cs = vlaw_alpha * std::log1p(static_cast<double>(raw_level[k2]) / vlaw_beta)
-                      + vlaw_c
-                      + (delta_mark[k2] ? vlaw_delta : 0.0);
-            band_level[k2] = static_cast<float>(std::pow(10.0, -cs / 20.0));
+            band_level[k2] = static_cast<float>(vlaw_mask(
+                static_cast<double>(raw_level[k2]), vlaw_alpha, vlaw_beta,
+                vlaw_c, delta_mark[k2] ? vlaw_delta : 0.0));
         }
         frame_dbg_ctr++;
     } else