prevent drawing separators on starting lines of fields

On non-consecutive ranges, we were drawing the starting line _on top_ of
the separator, which lead to some visual glitches in some viewers.

README.md

@@ -36,6 +36,6 @@ For more information, see the [manual](manual.pdf)
 To use this package, simply import `schema` from [rivet](https://typst.app/universe/package/rivet) and call `schema.load` to parse a schema description. Then use `schema.render` to render it, et voilà !
 ```typ
 #import "@preview/rivet:0.3.0": schema
-#let doc = schema.load("path/to/schema.yaml")
+#let doc = schema.load(yaml("path/to/schema.yaml"))
 #schema.render(doc)
 ```

manual.typ

@@ -152,6 +152,34 @@ The range name (or key) defines the left- and rightmost bits (e.g. `7-4` goes fr
 }
 ```
 
+=== Start <format-start>
+
+By default, structures start at bit 0, but you may want to number bits from 1, or another arbitrary value. To do this, you can set the `start` property of a structure to the desired start value. For example,
+
+```json
+"main": {
+  "bits": 8,
+  "start": 4,
+  "ranges": {
+    "11-7": { ... },
+    "6-4": { ... }
+  }
+}
+```
+
+#let start-schema = (structures: (main: (bits: 8, start: 4, ranges: ("11-7": (name: ""), "6-4": (name: "")))))
+
+would render as
+
+#align(
+  center,
+  schema.render(
+    schema.load(start-schema),
+    width: 50%
+  )
+)
+
+
 == Range <format-range>
 
 A range represents a group of consecutive bits. It can have a name (displayed in the bit cells), a description (displayed under the structure) and / or values.

src/renderer.typ

@@ -387,8 +387,10 @@
   }
   let range-boundaries = ()
   for r in struct.ranges.values() {
-    let i = to-real-i(if config.ltr-bits {r.start} else {r.end})
-    range-boundaries.push(i)
+    let start-i = to-real-i(if config.ltr-bits {r.start} else {r.end})
+    let end-i = to-real-i(if config.ltr-bits {r.end} else {r.start}) + 1
+    range-boundaries.push(start-i)
+    range-boundaries.push(end-i)
   }
 
   // Draw colors
@@ -407,6 +409,11 @@
       indices.push(r.start)
       indices.push(r.end)
     }
+    // ensure first and last bits are included
+    if not indices.contains(0) { indices.insert(0, struct.start) }
+    if not indices.contains(struct.bits + struct.start - 1) {
+      indices.push(struct.bits + struct.start - 1)
+    }
   }
 
   for i in range(struct.bits) {
@@ -453,8 +460,9 @@
   let desc-y = bits-y + bit-h * 2
 
   // Names + simple descriptions
-  for range_ in ranges {
+  for (i, range_) in ranges.enumerate() {
     let start-i = to-real-i(if config.ltr-bits {range_.start} else {range_.end})
+    let end-i = to-real-i(if config.ltr-bits {range_.end} else {range_.start})
     let start-x = bits-x + start-i * bit-w
     let width = rng.bits(range_) * bit-w
 
@@ -465,6 +473,28 @@
     shapes += draw-line(border-col, (line-x, bits-y), (line-x, bits-y + bit-h))
     shapes += draw-text(range_.name, txt-col, name-x, name-y, fill: bg-col)
 
+    // paint end line only if needed
+    if (
+      start-i != 0
+        and end-i != struct.bits - 1
+        and ranges
+          .at(
+            i
+              + {
+                if config.ltr-bits and i < ranges.len() - 1 { 1 } else { -1 }
+              },
+          )
+          .end
+          != range_.start - 1
+    ) {
+      line-x += if config.ltr-bits { -width } else { width }
+      shapes += draw-line(
+        border-col,
+        (line-x, bits-y),
+        (line-x, bits-y + bit-h),
+      )
+    }
+    
     if range_.description != "" {
       let shapes_
       (shapes_, desc-x, desc-y) = draw-description(