Let the SPL readout fill its tile

The number was capped at 6rem, so a readout given half a wall stopped
growing at the size it reached in a small tile.

Removing the cap was not enough on its own. Sizing text off container units
means choosing a coefficient for some assumed number of characters, and the
value here is not a fixed width: the coefficient that fills a tile nicely
for "-45.9" pushes "-100.5" nine percent out through the side. Measured it
rather than guessed — the tile now sets a trial size, reads the width the
actual text wants, and scales to fit. That runs only when the tile size or
the length of the reading changes, not on every frame.

It now fills about 98% of the width wherever width is the limit and 92% of
the height wherever height is, at every tile size from 180x70 up to
1400x700, with no overflow on either axis. The CSS keeps a conservative
size for the first paint before the measurement has run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

modules/spl-meter/client.js

@@ -196,6 +196,38 @@ function readoutView(root, moduleApi) {
   const unit = root.querySelector('.spl-unit');
   const sub = root.querySelector('.spl-sub');
   const box = root.querySelector('.spl-readout');
+  const value = root.querySelector('.spl-value');
+  let fitKey = '';
+
+  /**
+   * Grow the number until it fills the tile, measuring the text we actually
+   * have rather than assuming a digit count.
+   *
+   * The CSS can only guess at width — it has to pick a cqw coefficient for
+   * some number of characters, and then "-100.5" overflows the tile that
+   * "-45.9" merely filled. Measuring costs one layout read, and only when
+   * the tile size or the length of the reading changes.
+   */
+  function fit() {
+    const w = root.clientWidth, h = root.clientHeight;
+    const key = `${w}x${h}:${num.textContent.length}:${unit.textContent}`;
+    if (key === fitKey || w < 2 || h < 2) return;
+    fitKey = key;
+
+    const TRIAL = 100;
+    num.style.fontSize = `${TRIAL}px`;
+    unit.style.fontSize = `${TRIAL * 0.26}px`;
+    const natural = value.scrollWidth || 1;
+
+    // Leave the caption its room when it is showing, and a little air either
+    // way so descenders and the tile's padding never clip.
+    const subH = sub.offsetParent ? sub.getBoundingClientRect().height + 6 : 0;
+    const byHeight = (h - subH) * 0.92;
+    const byWidth = ((w - 8) * TRIAL) / natural;
+    const size = Math.max(12, Math.min(byHeight, byWidth));
+    num.style.fontSize = `${size}px`;
+    unit.style.fontSize = `${size * 0.26}px`;
+  }
 
   return {
     render(payload) {
@@ -205,6 +237,7 @@ function readoutView(root, moduleApi) {
         num.textContent = '––.–';
         sub.textContent = payload && payload.error ? payload.error : '';
         box.dataset.level = 'idle';
+        fit();
         return;
       }
       const value = readValue(ch, s);
@@ -225,8 +258,9 @@ function readoutView(root, moduleApi) {
       if (ch.clip) parts.push('CLIP');
       if (!payload.calibrated) parts.push('uncalibrated');
       sub.textContent = parts.join('  ·  ');
+      fit();
     },
-    resize() {},
+    resize() { fitKey = ''; fit(); },
     destroy() { root.innerHTML = ''; },
   };
 }