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.

src/core/ui/component_geometry.cpp

@@ -102,7 +102,13 @@ WaveformBand waveformBand(int bandTop, int bandHeight) {
     b.top = bandTop;
     b.height = bandHeight;
     b.midY = bandTop + bandHeight / 2;
-    b.halfSpan = (bandHeight / 2) - 2;  // matches drawWaveform's 2px vertical breathing room
+    // matches drawWaveform's 2px vertical breathing room; clamped at 0 so a degenerate band
+    // (height <= 3, where this would otherwise go negative) can't flip a positive peak below
+    // the zero line.
+    const int rawHalfSpan = (bandHeight / 2) - 2;
+    b.halfSpan = rawHalfSpan > 0 ? rawHalfSpan : 0;
+    // lo/hi (waveformColumnSpan) are midY-height/2 .. midY+height/2-1 — asymmetric by one px for
+    // even heights. Unreachable while |compressAmplitudeForDisplay| <= 1, so left as-is.
     return b;
 }