feat(keyboard_strip): add isNaturalKey predicate for piano-pattern overlay (S-VIEW-7)

src/vst/keyboard_strip.h

@@ -118,4 +118,14 @@ ZoneBarHit zoneBarAtPoint(const StripLayout& layout, const int* lows, const int*
 // key centre. Returns startNote unchanged for dxPixels==0.
 int resolveDragNote(const StripLayout& layout, int startNote, int dxPixels);
 
+// Returns true when `note` (0..127) is a NATURAL (white) key in standard 12-tone equal
+// temperament; false when it is an ACCIDENTAL (black) key. Notes out of the [0,127]
+// range are clamped to [0,127] before classification (i.e. this never throws/UBs on a
+// bad input). The 12 semitone positions within an octave:
+//   Natural (white):    0(C)  2(D)  4(E)  5(F)  7(G)  9(A)  11(B)
+//   Accidental (black): 1(C#) 3(D#) 6(F#) 8(G#) 10(A#)
+// Used by the shell to overlay the two-tone bright/dark piano-key pattern over the
+// pastel spectral fill (S-VIEW-7). Pure — no layout required, no host types.
+bool isNaturalKey(int note);
+
 }  // namespace reasampler::vst