Fences join like walls, fence designs, draw runs corner by corner

Joints: line_joint (util.rhai) applies the house-wall rules to fences and
site walls, so pieces never overlap and the depth sort stays clean.
- A corner is closed: the thicker piece (or, if equal, the one along
  world x) runs through, and the other stops at its face.
- On a straight run, two pieces share one post.
- At a T, the stem stops at the other piece's face without a post of its
  own.
- A fence that ends at a site or house wall stops at the wall's face.

Fence designs (prop style): mesh, wooden boards, steel bars, mesh on a
stone base, cobble wall, and brick wall with pillars. Board, stone and
brick joints are drawn on the face turned to the viewer. Brick pillars
are separate sprites.

Studio: line prefabs (walls, fences, hedges, roads) can be drawn as a
run by clicking corner after corner. Double-click or Enter ends the run,
clicking the first corner closes it, and Backspace drops the last
corner. A run is one undo step. A drag still draws one piece. Corners
and ends snap onto the ends and lines of the pieces they meet, and a
marker shows where the next corner will snap.

The house demo gets a brick front wall with gate pillars and wooden side
fences in place of the old east fence, which ran over the road. Tests
cover every design's corners, Ts, straight runs and wall stops, with no
depth-sort cycles in any rotation, plus site walls. check_studio draws
and closes a fence run, undoes it in one step, snaps a dragged spur,
and picks a design.

README.md

@@ -46,7 +46,7 @@ Try the demos: `isokit studio examples/demo` (a works site) and
 | Do | How |
 |---|---|
 | Add | pick an object on the left, click on the map (Shift-click places several) |
-| Draw | walls, roads, fences, hedges: drag along the map; floors, areas, buildings, roofs, rugs: drag a rectangle |
+| Draw | walls, fences, hedges, roads: drag along the map, or click corner after corner for a run around a plot (double-click / Enter ends it, the first corner closes it); ends snap onto the walls and fences they meet. Floors, areas, buildings, roofs, rugs: drag a rectangle |
 | Select | click it on the map or in the object list |
 | Move | drag it, or arrow keys by the snap step (0.1 / 0.5 / 1 / 5 m) |
 | Turn | R / Shift+R or ⟲ ⟳ |
@@ -72,6 +72,12 @@ The **House** palette has the parts, **Furniture** the interior:
 4. **Roof** (hip, gable, flat, optional solar) as a rectangle over the walls,
    and **Stairs**.
 
+**Fences and walls** around the plot: the Fence has six designs (mesh, wooden
+boards, steel bars, mesh on a stone base, cobble wall, brick wall with
+pillars). Fences and site walls whose ends meet join like house walls: corners
+close, a T stops at the other's face, a straight run shares one post, and a
+fence ending at a wall stops at its face.
+
 Level **GF** shows the house cut open: walls in front of a room are cut low
 (walls tell inside from outside by the floors next to them), the roof becomes an
 outline on the ground. For more storeys, view **1F** (2F, …): new walls, floors