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_knob_deck.cpp

@@ -382,6 +382,27 @@ static void testKnobFaceResolvesInnerRingOuterRingAndMisses() {
     CHECK(hitTestKnobFace(dl, cx + diag, cy + diag).id == -1);
 }
 
+// A non-square knob rect (e.g. a squashed chrome row clamps knob height below its width) must
+// resolve against min(width, height)/2 — the same radius computeKnob draws — never against
+// width alone, or the hit disc would claim territory above/below where nothing is drawn.
+static void testInKnobFaceUsesTheSmallerDimensionOnANonSquareRect() {
+    const Rect wide{0, 0, 40, 20};  // width > height: draws a 10px-radius disc, not 20px
+    const int cx = wide.x + wide.width / 2;
+    const int cy = wide.y + wide.height / 2;
+    CHECK(inKnobFace(wide, cx, cy));           // dead centre always hits
+    CHECK(inKnobFace(wide, cx, cy + 9));       // just inside the drawn (height-limited) radius
+    CHECK(!inKnobFace(wide, cx, cy + 10));     // on the drawn rim: exclusive miss
+    CHECK(!inKnobFace(wide, cx, cy + 15));     // inside the RECT but outside the smaller-radius disc
+    CHECK(!inKnobFace(wide, cx + 15, cy));     // same check along the wider axis
+
+    const Rect tall{0, 0, 20, 40};  // height > width: draws a 10px-radius disc, not 20px
+    const int tcx = tall.x + tall.width / 2;
+    const int tcy = tall.y + tall.height / 2;
+    CHECK(inKnobFace(tall, tcx, tcy));
+    CHECK(!inKnobFace(tall, tcx + 15, tcy));
+    CHECK(!inKnobFace(tall, tcx, tcy + 15));
+}
+
 static void testEmptyDeck() {
     const std::vector<DeckGroupDesc> none;
     CHECK(deckRowCount(none, 800) == 0);
@@ -401,6 +422,7 @@ int main() {
     testCaptionRadioGeometryAndHit();
     testInnerDialHit();
     testKnobFaceResolvesInnerRingOuterRingAndMisses();
+    testInKnobFaceUsesTheSmallerDimensionOnANonSquareRect();
     testCaptionToggle2();
     testEmptyDeck();
     if (g_fail) {