fix: close five Θ-W6-T1 review minors — headroom figure, knob-face radius, degenerate band clamp

Aligns inKnobFace's hit radius with computeKnob's draw-side min(w,h) rule and adds a non-square-rect test; clamps halfSpan for degenerate waveform bands with a test; fixes stale docs/comments and annotates an uncommitted perf measurement.

tests/test_component_geometry.cpp

@@ -197,7 +197,9 @@ static void testWaveformColumnCount() {
 // The regression this exists to catch: rounding applied to the resulting y instead of to the
 // scaled amplitude draws a symmetric column one pixel taller above the zero line than below.
 static void testSymmetricColumnDrawsEqualHeightAboveAndBelowTheZeroLine() {
-    const WaveformBand band = waveformBand(0, 41);  // odd height -> fractional half-span
+    // Odd height so midY sits equidistant from lo/hi — the fractional half-span below comes
+    // from the amplitude product, not the height's parity.
+    const WaveformBand band = waveformBand(0, 41);
     // Sweep amplitudes whose scaled value is fractional, which is where the two edges can
     // round in opposite directions.
     for (int i = 1; i <= 20; ++i) {
@@ -256,6 +258,18 @@ static void testBandMetricsMirrorTheDrawnInset() {
     CHECK(b.halfSpan == 18.0);  // half the band, less the 2px breathing room
 }
 
+// A band too short for the 2px breathing room (height <= 4, so height/2 - 2 <= 0) must clamp
+// halfSpan to 0 rather than go negative and mirror every positive column below the zero line.
+static void testDegenerateBandClampsHalfSpanToZero() {
+    CHECK(waveformBand(0, 4).halfSpan == 0.0);
+    CHECK(waveformBand(0, 3).halfSpan == 0.0);
+    CHECK(waveformBand(0, 0).halfSpan == 0.0);
+    // A zero half-span collapses every column onto the zero line regardless of amplitude sign.
+    const WaveformBand b = waveformBand(10, 4);
+    const WaveformColumnSpan s = waveformColumnSpan(b, 1.0, -1.0);
+    CHECK(s.top == b.midY && s.bottom == b.midY);
+}
+
 int main() {
     testHitTestBoxHalfOpen();
     testButtonBoxInset();
@@ -279,6 +293,7 @@ int main() {
     testTallerAmplitudeNeverDrawsAShorterColumn();
     testBothEdgesAndTheStrokeClampToTheBand();
     testBandMetricsMirrorTheDrawnInset();
+    testDegenerateBandClampsHalfSpanToZero();
 
     if (g_fail == 0) std::printf("component_geometry: all tests passed\n");
     else std::printf("component_geometry: %d CHECK(s) FAILED\n", g_fail);