sandbox/studio: fix amber arc clipping at the canvas edges

The amber arc was drawn at radius ~30px from center on a 56×56
canvas — the bottom of the arc (at angles near 6 o'clock) sat at
y ≈ 58 and got clipped by the canvas's own bounding box. Visible
as a flat cut-off at extreme positions.

Fix: bump the canvas to 72×72 and use fixed-pixel values for the
knob's geometry instead of fractions of `size`, so the visible knob
diameter stays at ~52px regardless of canvas size and there's now
~10px of margin all around for the arc and drop shadow.

- KNOB_SIZE: 56 → 72
- totalR: size * 0.46 → 26 (fixed)
- rimW:   size * 0.045 → 2.5 (fixed)
- notch half-base: size * 0.026 → 1.5 (fixed)
- cy: size/2 + 1 → size/2 (no need for the offset on the larger canvas)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/sandbox/studio.html

@@ -575,7 +575,9 @@ for (const line of PATCH) {
 //  knob — restrained: dark gradient + thin amber arc + single tickmark.
 //  No knurled ridges, no brushed metal. Closer to the reference image.
 // ===========================================================================
-const KNOB_SIZE = 56;
+// Canvas is bigger than the visible knob so the amber arc + drop shadow have
+// room around the dial. Visible knob radius is ~26px regardless of canvas size.
+const KNOB_SIZE = 72;
 function setupCanvas(canvas, size) {
   const dpr = window.devicePixelRatio || 1;
   canvas.style.width  = size + 'px';
@@ -588,9 +590,11 @@ function setupCanvas(canvas, size) {
 }
 
 function drawKnob(ctx, size, norm) {
-  const cx = size / 2, cy = size / 2 + 1;
-  const totalR = size * 0.46;          // outer edge of the whole knob
-  const rimW   = size * 0.045;         // dark rim thickness (thin)
+  const cx = size / 2, cy = size / 2;
+  // Fixed pixel sizes so the visible knob doesn't grow with the canvas.
+  // Canvas size only adds margin around these for arc + shadow.
+  const totalR = 26;                    // outer edge of the whole knob
+  const rimW   = 2.5;                   // dark rim thickness
   const discR  = totalR - rimW;         // metallic disc fills most of inside
   ctx.clearRect(0, 0, size, size);
 
@@ -723,7 +727,7 @@ function drawKnob(ctx, size, norm) {
   const tipR  = discR * 0.86;
   const baseR = discR * 0.50;
   const perp  = ang + Math.PI / 2;
-  const half  = size * 0.026;
+  const half  = 1.5;                    // fixed half-width of the notch base
   const tipX = cx + Math.cos(ang) * tipR;
   const tipY = cy + Math.sin(ang) * tipR;
   const bX   = cx + Math.cos(ang) * baseR;