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.

crates/isokit-script/stdlib/nature.rhai

@@ -47,7 +47,15 @@ fn meta_hedge() {
 }
 fn prefab_hedge(o) {
     let p = o.p;
-    sprite().box(-p.w / 2.0, -p.d / 2.0, p.w / 2.0, p.d / 2.0, 0.0, p.h, "hedge");
+    let c0 = hedge_end(o, -1.0);
+    let c1 = hedge_end(o, 1.0);
+    sprite().cut_box(c0.x, c0.k, c1.x, c1.k, -p.d / 2.0, p.d / 2.0, 0.0, p.h, "hedge");
+}
+
+/// Where hedge o ends at e: mitred or slanted where a diagonal hedge meets it.
+fn hedge_end(o, e) {
+    let m = line_meet(o, e, ["hedge"], []);
+    if m.kind == "corner" || m.kind == "tee" { end_cut(o.p.w, e, m) } else { end_cut(o.p.w, e, #{ ext: 0.0, k: 0.0 }) }
 }
 
 // ---------------------------------------------------------------- pond