feat: Add anchor-based zigzag bend placement via zigzag-at

Introduce a new `zigzag-at` parameter on `wire.wire` as an alternative
to `zigzag-ratio` for positioning the middle bend segment of a
`style: "zigzag"` wire. Instead of a fraction of the endpoint span,
the bend is placed on the axis of a referenced anchor via CeTZ's
perpendicular operator (`|-`):

  - zigzag-dir "vertical":   segment sits at anchor.x
  - zigzag-dir "horizontal": segment sits at anchor.y

Accepted forms:
  - str                         — raw CeTZ anchor name (e.g. port anchor
                                  "elmt-port-id" or "elmt.center")
  - array / native CeTZ dict    — used as-is
  - (rel: <length>, to: <coord>) with scalar rel — ergonomic axial
    offset, rewritten to (rel: (dx, 0), to: ...) for vertical dir or
    (rel: (0, dy), to: ...) for horizontal dir

`zigzag-ratio` default flipped from `50%` to `auto` (still resolves
to 50%) to allow detecting an explicit user override. Panics on
misuse:
  - `zigzag-at` set together with an explicit `zigzag-ratio`
  - `zigzag-at` set with a non-zigzag `style`

Add `examples.zigzag-at` demonstrating both the string-anchor and the
ergonomic offset forms.

doc/examples.typ

@@ -447,3 +447,24 @@ element.block(x: 18.5, y: -3.5, w: 1.5, h: 1.5, id: "sB",
 wire.wire("uSS", ("sA-port-out", "sB-port-out"),
           style: "u-shape", u-side: "south", u-depth: 0.5)
 ```, vertical: true)
+
+#let zigzag-at = example(```
+// `zigzag-at` places the middle bend of a zigzag wire
+// on the axis of a referenced anchor, overriding
+// `zigzag-ratio`.
+element.block(x: 0, y: 0, w: 1.5, h: 1.5, id: "src",
+  ports: (east: ((id: "o", name: "O"),),))
+element.block(x: 3, y: -1.2, w: 1, h: 1, id: "ref",
+  fill: util.colors.pink,
+  ports: (west: ((id: "p", name: "R"),),))
+element.block(x: 5.5, y: -2, w: 1.5, h: 1.5, id: "dst",
+  ports: (west: ((id: "i", name: "I"),),))
+// String form: anchor name of a port.
+wire.wire("w1", ("src-port-o", "dst-port-i"),
+          style: "zigzag",
+          zigzag-at: "ref-port-p")
+// Ergonomic offset: shift 8pt right of ref.east.
+wire.wire("w2", ("src-port-o", "dst-port-i"),
+          style: "zigzag",
+          zigzag-at: (rel: 8pt, to: "ref.east"))
+```, vertical: true)

src/wire.typ

@@ -27,29 +27,80 @@
   return (pts, anchors)
 }
 
-#let get-zigzag-wire(pts, ratio, dir) = {
+// Normalize a `zigzag-at` value.
+//
+// Accepted forms:
+//   - `none`                              → unset; ratio-based placement is used.
+//   - `str` / tuple / native CeTZ dict    → passed through as a CeTZ coordinate.
+//   - `(rel: <length>, to: <coord>)` with a *scalar* `rel`
+//                                         → ergonomic axial offset, rewritten
+//                                           to `(rel: (rel, 0), to: to)` for
+//                                           `dir == "vertical"` or
+//                                           `(rel: (0, rel), to: to)` for
+//                                           `dir == "horizontal"`. If `rel` is
+//                                           already an array/tuple, the value
+//                                           is passed through unchanged.
+#let normalize-zigzag-at(at, dir) = {
+  if at == none { return none }
+  if type(at) == dictionary and "rel" in at and "to" in at and type(at.rel) != array {
+    let r = at.rel
+    if dir == "vertical" {
+      return (rel: (r, 0), to: at.to)
+    } else {
+      return (rel: (0, r), to: at.to)
+    }
+  }
+  return at
+}
+
+#let get-zigzag-wire(pts, ratio, dir, at: none) = {
   let start = pts.first()
   let end = pts.last()
-  let mid = if dir == "vertical" {
-    (start, ratio, (horizontal: end, vertical: ()))
-  } else {
-    (start, ratio, (horizontal: (), vertical: end))
-  }
 
-  let points = if dir == "vertical" {
-    (
-      start,
-      (horizontal: mid, vertical: ()),
-      (horizontal: (), vertical: end),
-      end
-    )
+  let points
+  if at == none {
+    // Ratio-based placement (default).
+    let mid = if dir == "vertical" {
+      (start, ratio, (horizontal: end, vertical: ()))
+    } else {
+      (start, ratio, (horizontal: (), vertical: end))
+    }
+
+    points = if dir == "vertical" {
+      (
+        start,
+        (horizontal: mid, vertical: ()),
+        (horizontal: (), vertical: end),
+        end
+      )
+    } else {
+      (
+        start,
+        (horizontal: (), vertical: mid),
+        (horizontal: end, vertical: ()),
+        end
+      )
+    }
   } else {
-    (
-      start,
-      (horizontal: (), vertical: mid),
-      (horizontal: end, vertical: ()),
-      end
-    )
+    // Anchor-based placement: the middle bend segment sits on the axis
+    // extracted from `at` via CeTZ's perpendicular operator `|-`:
+    //   - vertical:   middle segment at x = at.x
+    //   - horizontal: middle segment at y = at.y
+    points = if dir == "vertical" {
+      (
+        start,
+        (at, "|-", start),
+        (at, "|-", end),
+        end
+      )
+    } else {
+      (
+        start,
+        (start, "|-", at),
+        (end, "|-", at),
+        end
+      )
+    }
   }
   let anchors = (
     "start": start,
@@ -301,8 +352,9 @@
 /// - reverse (bool): If true, the start and end points will be swapped (useful in cases where the start point depends on the end point, for example with perpendiculars)
 /// - directed (bool): If true, the wire will be directed, meaning an arrow will be drawn at the endpoint
 /// - rotate-name (bool): If true, the name will be rotated according to the wire's slope
-/// - zigzag-ratio (ratio): Position of the zigzag vertical relative to the horizontal span (only with style "zigzag")
+/// - zigzag-ratio (auto, ratio): Position of the zigzag middle bend segment relative to the horizontal (or vertical) span, as a fraction between the two endpoints (only with style "zigzag"). Defaults to `auto`, which resolves to `50%`. Panics if combined with an explicitly set `zigzag-at`.
 /// - zigzag-dir (str): The zigzag's direction. As either "vertical" or "horizontal" (only with dstyle "zigzag")
+/// - zigzag-at (none, str, array, dictionary): Optional. Anchor-based placement of the zigzag middle bend segment (only with style "zigzag"). When set, overrides `zigzag-ratio` (which must be left at its default `auto`). The middle segment sits on the axis extracted from the given coordinate: `x` for `zigzag-dir: "vertical"`, `y` for `"horizontal"`. Accepted forms: a raw CeTZ anchor name string (e.g. `"elmt.center"` or `"elmt-port-a"`); any native CeTZ coordinate (tuple, dict such as `(rel: (dx, dy), to: anchor)`); or the ergonomic scalar-offset form `(rel: <length>, to: <coord>)`, which is rewritten to a `(dx, 0)` offset for `"vertical"` and `(0, dy)` for `"horizontal"`. See #examples.zigzag-at
 /// - dodge-y (number): Y position to dodge the wire to (only with style "dodge")
 /// - dodge-sides (array): The start and end sides (going out of the connected element) of the wire (only with style "dodge")
 /// - dodge-margins (array): The start and end margins (i.e. space before dodging) of the wire (only with style "dodge")
@@ -326,8 +378,9 @@
   guided-sides: ("east", "west"),
   directed: false,
   rotate-name: true,
-  zigzag-ratio: 50%,
+  zigzag-ratio: auto,
   zigzag-dir: "vertical",
+  zigzag-at: none,
   dodge-y: 0,
   dodge-sides: ("east", "west"),
   dodge-margins: (5%, 5%),
@@ -342,6 +395,19 @@
     panic("Wrong number of points (got " + str(pts.len()) + " instead of 2)")
   }
 
+  // `zigzag-at` validation.
+  if zigzag-at != none {
+    if style != "zigzag" {
+      panic("wire '" + id + "': `zigzag-at` requires style: \"zigzag\" (got \""
+        + style + "\")")
+    }
+    if zigzag-ratio != auto {
+      panic("wire '" + id
+        + "': cannot combine `zigzag-at` with an explicit `zigzag-ratio`")
+    }
+  }
+  let zigzag-ratio-effective = if zigzag-ratio == auto { 50% } else { zigzag-ratio }
+
   // Normalize user-provided coordinates: (x, y) -> (x, y, 0)
   let pts = pts.map(normalize-coord)
   
@@ -360,7 +426,12 @@
     (points, anchors) = get-direct-wire(pts)
     
   } else if style == "zigzag" {
-    (points, anchors) = get-zigzag-wire(pts, zigzag-ratio, zigzag-dir)
+    (points, anchors) = get-zigzag-wire(
+      pts,
+      zigzag-ratio-effective,
+      zigzag-dir,
+      at: normalize-zigzag-at(zigzag-at, zigzag-dir),
+    )
     
   } else if style == "dodge" {
     (points, anchors) = get-dodge-wire(