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

tests/test_keyboard_strip.cpp

@@ -11,7 +11,8 @@
 // classifying low-edge / high-edge / body and the narrow-bar midpoint split (low wins the
 // tie); zoneBarAtPoint first-match on overlap + null-list rejection; resolveDragNote rounding
 // to the nearest key at the key centre, clamping to [0,127], and the zero-delta / zero-width
-// no-ops.
+// no-ops; isNaturalKey across a full octave (C4..B4), at boundary notes 0 and 127, and with
+// out-of-range inputs that clamp to [0,127].
 
 #include "../src/vst/keyboard_strip.h"
 
@@ -179,6 +180,38 @@ static void testResolveDragClampsAndNoOps() {
     CHECK(resolveDragNote(Z, 60, 500) == 60);
 }
 
+// --- isNaturalKey -------------------------------------------------------------
+
+static void testIsNaturalKeyFullOctave() {
+    // Semitone positions 0..11 starting at C4 (MIDI 60):
+    //   C=60(nat) C#=61(acc) D=62(nat) D#=63(acc) E=64(nat) F=65(nat)
+    //   F#=66(acc) G=67(nat) G#=68(acc) A=69(nat) A#=70(acc) B=71(nat)
+    const bool expected[12] = {
+        true,  false, true,  false, true,  true,
+        false, true,  false, true,  false, true,
+    };
+    for (int i = 0; i < 12; ++i) {
+        CHECK(isNaturalKey(60 + i) == expected[i]);
+    }
+}
+
+static void testIsNaturalKeyBoundaryNotes() {
+    // Note 0 is C (natural); note 127 is G (natural); note 1 is C# (accidental).
+    CHECK(isNaturalKey(0)   == true);   // C0  — natural
+    CHECK(isNaturalKey(1)   == false);  // C#0 — accidental
+    CHECK(isNaturalKey(127) == true);   // G9  — natural (127 % 12 == 7)
+    CHECK(isNaturalKey(126) == false);  // F#9 — accidental (126 % 12 == 6)
+}
+
+static void testIsNaturalKeyOutOfRangeClamped() {
+    // Values outside [0,127] clamp to [0,127]; must not crash/UB.
+    // note -1 clamps to 0 (C, natural); note 128 clamps to 127 (G, natural).
+    CHECK(isNaturalKey(-1)  == true);
+    CHECK(isNaturalKey(128) == true);
+    CHECK(isNaturalKey(-100) == true);
+    CHECK(isNaturalKey(200)  == true);
+}
+
 static void testResolveDragProportionalNonDivisibleWidth() {
     // THE REVIEW FINDING: 544px / 128 = 4.25 (non-integer). Old uniform-keyW math used
     // keyW = 4 (floor), accumulating ~7 keys of drift at the far end. The proportional fix
@@ -221,6 +254,9 @@ int main() {
     testResolveDragRoundsToNearestKey();
     testResolveDragClampsAndNoOps();
     testResolveDragProportionalNonDivisibleWidth();
+    testIsNaturalKeyFullOctave();
+    testIsNaturalKeyBoundaryNotes();
+    testIsNaturalKeyOutOfRangeClamped();
 
     if (g_fail == 0) std::printf("keyboard_strip: all tests passed\n");
     return g_fail != 0;