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

src/vst/keyboard_strip.cpp

@@ -102,6 +102,29 @@ ZoneBarHit zoneBarAtPoint(const StripLayout& layout, const int* lows, const int*
     return ZoneBarHit{};  // on the band but on no bar
 }
 
+bool isNaturalKey(int note) {
+    // Clamp to the valid MIDI range before indexing.
+    const int n = note < 0 ? 0 : (note > kStripKeyCount - 1 ? kStripKeyCount - 1 : note);
+    // The 12-semitone pattern of natural (white) keys within an octave, starting at C:
+    //   positions 0(C) 2(D) 4(E) 5(F) 7(G) 9(A) 11(B) are natural;
+    //   positions 1(C#) 3(D#) 6(F#) 8(G#) 10(A#) are accidental.
+    static constexpr bool kNatural[12] = {
+        true,   // 0  C
+        false,  // 1  C#
+        true,   // 2  D
+        false,  // 3  D#
+        true,   // 4  E
+        true,   // 5  F
+        false,  // 6  F#
+        true,   // 7  G
+        false,  // 8  G#
+        true,   // 9  A
+        false,  // 10 A#
+        true,   // 11 B
+    };
+    return kNatural[n % 12];
+}
+
 int resolveDragNote(const StripLayout& layout, int startNote, int dxPixels) {
     if (dxPixels == 0) return clampNote(startNote);
     const int bandWidth = std::max(0, layout.keys.width());

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