Implement path clipping with exact Bézier curves

[hooyuser]

This PR builds on top of the path boolean work from #1091 and is the next step toward addressing #618.

It adds a new clip function to clip path drawables against a closed clipping region:

clip(
  clipping-region,
  body,
  mode: "inside",
  ...
)

Depending on mode ("inside" or "outside"), it keeps only the geometry inside or outside the clipping-region. The body parameter takes the objects you want to clip, and each path drawable inside it is clipped independently against clipping-region. Note that unlike paths in clipping-region, paths in body may contain both open and closed subpaths.

Take mode: "inside" as an example. Under the hood, the stroke and fill of each path drawable in body are handled separately:

• If its stroke is not none: subpaths (open or closed) are trimmed to keep only the portions inside the clipping region.
• If its fill is not none:
  • Its closed subpaths together define the filled area according to body-fill-rule. A boolean intersection between that area and the clipping-region produces a newly generated, fill-only path.
  • Its open subpaths do not participate in the Boolean operation.

mode: "outside" follows the same split: it retains the outside portions of stroked paths and uses a boolean difference for filled areas.

To avoid repeating logic from #1091, I also reorganized the relevant Typst and Rust code into shared Typst/Rust modules. This is purely an internal cleanup and doesn't change the behavior of the existing boolean function.

---

Example: string diagram of snake equation

Someone on Discord asked how to draw a string diagram and came up with a solution using boolean operations (discussion link). With clip, we now have a more straightforward way to achieve this kind of effect.

#cetz.canvas({
  import cetz.draw: *

  let padded-anchor(name, position) = cetz.draw.group(name: name, {
    import cetz.draw: *

    let padding = 0.3
    anchor("default", position)
    anchor("north", (position.at(0), position.at(1) + padding))
    anchor("south", (position.at(0), position.at(1) - padding))
    anchor("east", (position.at(0) + padding, position.at(1)))
    anchor("west", (position.at(0) - padding, position.at(1)))
  })

  let path = {
    line("F_in", (4, 2))
    bezier((4, 2), "epsilon.east", (4, 3))
    bezier("epsilon.west", (2, 2), (2, 3))
    bezier((2, 2), "eta.east", (2, 1))
    bezier("eta.west", (0, 2), (0, 1))
    line((0, 2), "F_out")
  }

  on-layer(10, {
    circle((1, 1), radius: 3mm, name: "eta", fill: white)
    content((), $eta$)

    circle((3, 3), radius: 3mm, name: "epsilon", fill: white)
    content((), $epsilon$)

    padded-anchor("F_in", (4, 0))
    content("F_in.south", $F$)

    padded-anchor("F_out", (0, 4))
    content("F_out.north", $F$)

    path
  })

  let background(fill) = {
    rect(
      (-0.5, 0),
      (4.5, 4),
      stroke: none,
      fill: fill,
    )
  }

  let aqua-region = {
    merge-path(
      {
        path
        line("F_out", (-0.5, 4), (-0.5, 0))
      },
      close: true,
    )
  }

  on-layer(0, {
    background(lime)
    clip(aqua-region, background(aqua))
  })
})