Fix crossfade loudness bump: apply db_boost to overlap as a whole

Applying gain to each side independently (old code) caused a ~+3 dB
loudness spike at every segment seam. Equal-power crossfade (cos/sin)
already sums to 1.0 at the midpoint; the boost should be applied to
the blended overlap as a unit, not to each fade arm separately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -260,7 +260,12 @@ def _build_segments(total_dur_s: float, window_s: float, crossfade_s: float) ->
 def _cf_join(a: np.ndarray, b: np.ndarray,
              crossfade_s: float, db_boost: float, sr: int) -> np.ndarray:
     """Equal-power crossfade join.  Works for both mono (T,) and stereo (C, T) arrays.
-    Stereo arrays are expected in (channels, samples) layout."""
+    Stereo arrays are expected in (channels, samples) layout.
+
+    db_boost is applied to the overlap region as a whole (after blending), so
+    it compensates for the -3 dB equal-power dip without doubling amplitude.
+    Applying gain to each side independently (the common mistake) causes a
+    +3 dB loudness bump at the seam — this version avoids that."""
     stereo = a.ndim == 2
     n_a = a.shape[1] if stereo else len(a)
     n_b = b.shape[1] if stereo else len(b)
@@ -272,10 +277,11 @@ def _cf_join(a: np.ndarray, b: np.ndarray,
     fade_out = np.cos(t * np.pi / 2)   # 1 → 0
     fade_in  = np.sin(t * np.pi / 2)   # 0 → 1
     if stereo:
-        overlap = a[:, -cf:] * fade_out * gain + b[:, :cf] * fade_in * gain
+        # Blend first, then apply boost to the overlap region as a unit
+        overlap = (a[:, -cf:] * fade_out + b[:, :cf] * fade_in) * gain
         return np.concatenate([a[:, :-cf], overlap, b[:, cf:]], axis=1)
     else:
-        overlap = a[-cf:] * fade_out * gain + b[:cf] * fade_in * gain
+        overlap = (a[-cf:] * fade_out + b[:cf] * fade_in) * gain
         return np.concatenate([a[:-cf], overlap, b[cf:]])