fix: measure the description width instead of estimating it

With left-labels a description is placed by subtracting its width from
desc-x. That width was estimated as half the font size per character,
which undershoots Ubuntu Mono's advance of 0.56em by 12%, so the leader
line struck through the last letter of every label, as it does on the
manual's own cover page. Proportional fonts were off by more.

render is now a context block, so the text can be measured instead. The
font has to be passed explicitly: the set rule at the top of render is
not in scope for a measure further down the same context block.

src/renderer.typ

@@ -190,10 +190,15 @@
 
   if config.left-labels {
     // desc-x is the right edge of the label when they are on the left, so shift
-    // to its left edge: exactly the box width when wrapping, an estimate from
-    // the character count otherwise
+    // to its left edge
     txt-x -= if config.description-width == none {
-      range_.description.len() * config.default-font-size / 2pt
+      // the set rule in render is not in scope here, so measure the text with
+      // the font it will be drawn in
+      measure(text(
+        range_.description,
+        font: config.default-font-family,
+        size: config.default-font-size
+      )).width.pt()
     } else {
       config.description-width
     }