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/instrument/ui/knob_deck.cpp

@@ -193,7 +193,9 @@ DeckHit hitTestDeck(const DeckLayout& layout, int x, int y) {
 bool inKnobFace(const Rect& knob, int x, int y) {
     const double dx = x - (knob.x + knob.width / 2.0);
     const double dy = y - (knob.y + knob.height / 2.0);
-    const double r = knob.width / 2.0;
+    // Matches computeKnob's radius rule (param_slider.cpp) so the hit rule never claims more
+    // circle than is actually drawn when a caller's rect is non-square (e.g. a squashed chrome row).
+    const double r = (std::min)(knob.width, knob.height) / 2.0;
     return dx * dx + dy * dy < r * r;
 }