Add the `zigzag-at` parameter to place the middle bend of a zigzag wire
on the axis of a referenced anchor. It overrides `zigzag-ratio`, which
now defaults to `auto` (resolving to 50%) so the two are mutually
exclusive. Accepted forms are an anchor name, any CeTZ coordinate, or
the scalar offset `(rel: <length>, to: <coord>)`.
Add the generic helpers this relies on and that other wire styles reuse:
- `normalize-scalar-offset(at, axis)` turns a scalar offset into an x or
y offset; `normalize-zigzag-at` is a wrapper around it.
- `detect-port-side(anchor, ctx)` deduces the cardinal side a port faces
by comparing it with its element's `.center` anchor.
Add the `zigzag-at` example.
David Stadelmann
2c6e5f44df