fix(rst wheel): the S9+ ladder, not the S digit

Below 59+20 comes 59+15, then +10, +5, then a plain 59 and down through
58, 57. Stepping the S digit and leaving the suffix where it was turned
59+20 into 58+20, which nobody has ever said on the air. Five decibels at
a time above S9 — the same increment sMeterRST rounds an S-meter reading
to — and one S-unit below it.

frontend/src/lib/rst.ts

@@ -59,11 +59,30 @@ export function stepRST(value: string, dir: number, mode: string): string {
     const next = Math.max(-30, Math.min(35, n + dir));
     return (next < 0 ? '-' : '+') + String(Math.abs(next)).padStart(2, '0');
   }
-  // RST/RS: the S digit is the one that moves. R and T are judgements about
-  // readability and tone that a wheel has no business changing, and S is the
-  // figure an operator actually adjusts.
-  const m = /^(\d)(\d)(\d?)((?:\+\d+)?)$/.exec(v);
+  // RST/RS, and the S9+ ladder above it. What an operator counts through is
+  //
+  //   … 57  58  59  59+5  59+10  59+15  59+20 …
+  //
+  // — one S-unit up to nine, then five decibels at a time, because that is how
+  // a strong signal is reported and how the S-meter is read (see sMeterRST).
+  // Stepping the S digit and leaving the "+20" where it was would have gone
+  // from 59+20 to 58+20, which nobody has ever said on the air.
+  //
+  // R and T do not move: they are judgements about readability and tone, and a
+  // wheel has no business changing them.
+  const m = /^(\d)(\d)(\d?)(?:\+(\d+))?$/.exec(v);
   if (!m) return v;
-  const s = Math.max(1, Math.min(9, parseInt(m[2], 10) + dir));
-  return m[1] + String(s) + m[3] + m[4];
+  const head = m[1];
+  const tone = m[3];
+  let sUnit = parseInt(m[2], 10);
+  let over = m[4] === undefined ? 0 : parseInt(m[4], 10);
+  if (over > 0) {
+    // Above S9: five at a time, and back down through +5 to a plain 59.
+    over = Math.max(0, Math.min(60, over + dir * 5));
+  } else if (sUnit >= 9 && dir > 0) {
+    over = 5; // 59 → 59+5
+  } else {
+    sUnit = Math.max(1, Math.min(9, sUnit + dir));
+  }
+  return head + String(sUnit) + tone + (over > 0 ? '+' + over : '');
 }