Divergence meter: keep digits inside the tubes, close the domes

The tube was 56 px wide while its dome (radius 28) spans 57, so the dome
overshot the right wall by a pixel; tubes are now 57 px. The Nixie digits
at size 110 were up to 60 px wide (plus the 1 px glow) in a 55 px tube;
size 90 fits, and each digit is clipped to the inside of its glass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/el512/steins.cpp

@@ -98,7 +98,8 @@ constexpr int WORLD_LINE_COUNT = sizeof(WORLD_LINES) / sizeof(WORLD_LINES[0]);
 
 // One Nixie tube: glass outline, anode mesh, socket, and the lit digit.
 void tube(Display &it, const Fonts &f, int x, int y, char digit) {
-  constexpr int TW = 56, TH = 176, R = 28;
+  // Odd width so the dome (radius R, 2R + 1 pixels across) meets both walls.
+  constexpr int TW = 57, TH = 176, R = 28;
   const int cx = x + TW / 2;
   arc(it, cx, y + R, R, -90, 90);
   it.vertical_line(x, y + R, TH - R - 14);
@@ -111,9 +112,12 @@ void tube(Display &it, const Fonts &f, int x, int y, char digit) {
       it.draw_pixel_at(mx, my);
   if (digit != ' ') {
     const char s[2] = {digit, 0};
-    // Twice, one pixel apart, so the thin filament reads as glowing.
+    // Twice, one pixel apart, so the thin filament reads as glowing; kept
+    // inside the glass.
+    it.start_clipping(x + 2, y + 2, x + TW - 2, y + TH - 14);
     it.print(cx, y + TH / 2 - 8, f.nixie, TextAlign::CENTER, s);
     it.print(cx + 1, y + TH / 2 - 8, f.nixie, TextAlign::CENTER, s);
+    it.end_clipping();
   }
 }