Diagonals: streets, walls, fences, hedges and house walls at any angle

Object frames turn by any angle (quarter turns stay exact, so scenes along
the axes render byte for byte as before); a box in a turned frame is a
prism with its own corners. Rotation is kept as given (normalised to
0..360), no longer snapped to quarter turns.

Depth sort: sprites whose plan outline is not their box carry their convex
outline; when boxes overlap, an outline edge that separates two sprites
orders them. A long diagonal wall sorts right against both sides.

Joints (line_meet, end_cut, cut_box in util.rhai): where a piece off the
axes is involved, corners are mitred and T-junctions slanted; fences put a
corner post or pillar in its own sprite and mitre stone bases; Y joints let
the lowest id run through. Streets: every end part is a cut e + k*w, so
side streets come in at any angle (slanted sidewalk gaps), two streets meet
in a mitre with seam patches; fixes a curve's outer sidewalk that was
skipped when its kerb and back lines had different point counts.

Studio: runs and drags keep to x/y or snap to 45 degree diagonals, Ctrl
gives any angle; a road run turning 45 degrees gets a 45 degree curve;
Rotation is a number box. Streets demo: a diagonal lane with a hedge.
MCP guidance, SCRIPTING/README/ROADMAP.

Tests: Rust 76, Studio and web checks green.

README.md

@@ -48,11 +48,11 @@ Try the demos: `isokit studio examples/demo` (a works site),
 | Do | How |
 |---|---|
 | Add | pick an object on the left, click on the map (Shift-click places several) |
-| 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; a road run gets a curve in each corner. 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; a road run gets a curve in each corner. Pieces keep to x or y, or go diagonal at 45° once the pointer is more than 22.5° off; hold Ctrl for any angle Floors, areas, buildings, roofs, rugs: drag a rectangle |
 | Select | click it on the map or in the object list; click a wall or fence again to select every piece joined to it |
 | Several | Ctrl+click (map or list) adds or takes out one, Shift+click in the list a range, Shift+drag on the map a box, Ctrl+A all; then drag one to move them all, arrows, R, Ctrl+D, Delete and the property form work on all of them |
 | Move | drag it, or arrow keys by the snap step (0.1 / 0.5 / 1 / 5 m) |
-| Turn | R / Shift+R or ⟲ ⟳ |
+| Turn | R / Shift+R or ⟲ ⟳ (quarter turns), or type any angle in Rotation |
 | Edit | the right panel: size, storeys, colours, solar, tooltip text, label, levels |
 | Copy / delete | Ctrl+D / Delete; Ctrl+Z / Ctrl+Y undo and redo |
 | Look | drag the ground to pan, Ctrl+wheel or pinch to zoom, ↺ ↻ rotate, Site / GF / 1F levels |
@@ -210,6 +210,12 @@ give-way lines for traffic on the left or right. Roads meet by their centre
 lines, so put the ends on each other. `examples/streets` (Streets demo in the
 web Studio) is a small town quarter.
 
+Diagonals: roads, walls, fences, hedges and house walls run at any angle.
+Where they meet at an angle the corners are mitred, side streets come in at a
+slant (the sidewalk opens along them), fence corners get a post or pillar, and
+a road run turning 45° gets a 45° curve. Windows and doors sit on diagonal
+house walls like on any other.
+
 Animations (fans, smoke, fish, traffic) step on about ten times a second by
 default ("light"): running CSS animations repaint the whole drawing every frame,
 which keeps a slow laptop busy. The ▷ button in the view toolbar switches