feat: add description-width config option

Descriptions and values are drawn as a single line that never wraps.
Because a diagram is scaled to fit the width it is rendered into, the
longest label is what decides that scale: one long description makes the
whole diagram -- bit cells, indices and every other label with it --
shrink until that line fits. A register whose fields carry a sentence
each is then unreadable at any page width, and since each diagram is
measured on its own, a document full of them renders every one at a
different size.

Setting description-width wraps labels at that width instead, so the
length of a description no longer decides how large the diagram is
drawn. Values wrap at the same right edge as the description they belong
to, and the option composes with left-labels, force-descs-on-side and
ltr-bits.

Wrapping means a label is no longer one line tall, so labels can no
longer be spaced by a fixed line height. Each is now offset by the
height it actually occupies, measured before it is placed, which is why
render gained a context block.

The option defaults to none, which keeps the single-line behaviour on
the original code paths. With it unset every diagram in the gallery
renders byte-identical to before.

docs/config.typ

@@ -26,7 +26,7 @@
 /// - full-page (bool): If true, the page will be resized to fit the diagram and take the background color
 /// - all-bit-i (bool): If true, all bit indices will be rendered, otherwise, only the ends of each range will be displayed
 /// - ltr-bits (bool): If true, bits are placed with the LSB on the left instead of the right
-/// - description-width (float, none): If set, descriptions and values wrap at this width instead of being drawn on a single line
+/// - description-width (float | none): If set, descriptions and values wrap at this width instead of being drawn on a single line
 /// -> dictionary
 #let config(
   default-font-family: "Ubuntu Mono",

manual.typ

@@ -203,6 +203,36 @@ For values depending on other ranges, see #link(<format-dependencies>)[Dependenc
 }
 ```
 
+=== Wrapping descriptions <format-description-width>
+
+Descriptions and values are drawn on a single line. A diagram is scaled to fit
+the width it is given, so a long description makes the whole diagram, bit cells
+and all, shrink until it fits:
+
+#let long-schema = (structures: (main: (bits: 16, ranges: (
+  "15-8": (name: "op", description: "Operation to perform on the two operands"),
+  "7-0": (
+    name: "flags",
+    description: "Flags applied to the operation, see the table below for the meaning of each bit",
+    values: ("00000001": "carry in, added to the result before the flags are evaluated")
+  )
+))))
+#let long-sch = schema.load(long-schema)
+
+#align(center, schema.render(long-sch, width: 75%))
+
+Setting the #doc-ref("config.config") option `description-width` wraps them at
+that width instead, leaving the diagram at its natural size. The width is given
+in the same units as `bit-width`, and values wrap at the same right edge as the
+description they belong to. Each label is offset by the height it actually
+occupies, so wrapping onto several lines does not make labels overlap.
+
+```typ
+schema.render(sch, config: config.config(description-width: 300))
+```
+
+#align(center, schema.render(long-sch, width: 75%, config: lib.config.config(description-width: 300)))
+
 #pagebreak(weak: true)
 
 == Dependencies <format-dependencies>
@@ -377,9 +407,7 @@ structures:
 
 #pagebreak(weak: true)
 
-= Config
-
-== Presets
+= Config presets
 
 Aside from the default config, some example presets are also provided:
 - #doc-ref("config.config", full: true): the default theme, black on white
@@ -391,38 +419,6 @@ Aside from the default config, some example presets are also provided:
 
 #pagebreak(weak: true)
 
-== Wrapping descriptions <config-description-width>
-
-Descriptions and values are drawn on a single line. A diagram is scaled to fit
-the width it is given, so a long description makes the whole diagram, bit cells
-and all, shrink until it fits:
-
-#let long-schema = (structures: (main: (bits: 16, ranges: (
-  "15-8": (name: "op", description: "Operation to perform on the two operands"),
-  "7-0": (
-    name: "flags",
-    description: "Flags applied to the operation, see the table below for the meaning of each bit",
-    values: ("00000001": "carry in, added to the result before the flags are evaluated")
-  )
-))))
-#let long-sch = schema.load(long-schema)
-
-#align(center, schema.render(long-sch, width: 75%))
-
-Setting the #doc-ref("config.config") option `description-width` wraps them at
-that width instead, leaving the diagram at its natural size. The width is given
-in the same units as `bit-width`, and values wrap at the same right edge as the
-description they belong to. Each label is offset by the height it actually
-occupies, so wrapping onto several lines does not make labels overlap.
-
-```typ
-#schema.render(sch, config: config.config(description-width: 300))
-```
-
-#align(center, schema.render(long-sch, width: 75%, config: lib.config.config(description-width: 300)))
-
-#pagebreak(weak: true)
-
 = Reference
 
 #let doc-config = tidy.parse-module(

src/renderer.typ

@@ -190,15 +190,10 @@
 
   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
+    // to its left edge: exactly the box width when wrapping, an estimate from
+    // the character count otherwise
     txt-x -= if config.description-width == none {
-      // 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()
+      range_.description.len() * config.default-font-size / 2pt
     } else {
       config.description-width
     }