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.

examples/streets/scene.json

@@ -10,6 +10,8 @@
     {"id": "bend", "type": "road_curve", "name": "Corner", "x": 0, "y": 34, "rot": 90, "props": {"walk": "both", "walk_w": 2.5, "d": 6.5, "r": 12, "turn": "right"}},
     {"id": "park-rd", "type": "road", "name": "Park road", "x": 34, "y": 46, "props": {"walk": "both", "walk_w": 2.5, "d": 6.5, "w": 44}},
     {"id": "cross-st", "type": "road", "name": "Cross street", "x": 34, "y": 23, "rot": 90, "props": {"w": 46, "d": 5.5, "walk": "both", "walk_w": 2}},
+    {"id": "lane", "type": "road", "name": "Garden lane", "x": 34.0, "y": -10.0, "rot": 315.0, "props": {"w": 28.284, "d": 5.5, "walk": "both", "walk_w": 2}},
+    {"id": "hedge-lane", "type": "hedge", "name": "Lane hedge", "x": 28.95, "y": -14.849, "rot": 315.0, "props": {"w": 12.0, "d": 0.8, "h": 1.2}},
     {"id": "offices", "type": "building", "name": "Offices", "x": 19, "y": 27, "props": {"w": 14, "d": 13, "floors": 3, "pv": true}},
     {"id": "flats", "type": "building", "name": "Flats", "x": 46, "y": 23, "props": {"w": 10, "d": 28, "floors": 4, "roof": "gable", "mat": "brick"}},
     {"id": "school", "type": "building", "name": "School", "x": -33, "y": -21, "props": {"w": 20, "d": 14, "floors": 2, "ac": true}},
@@ -30,7 +32,7 @@
     {"id": "walker-2", "type": "person", "x": -26, "y": -4.7},
     {"id": "walker-3", "type": "person", "x": 31.6, "y": 30, "props": {"cls": "guest3"}},
     {"id": "crowd", "type": "crowd", "name": "Pupils", "x": -24, "y": -11, "props": {"n": 6, "r": 1.6}},
-    {"id": "lamp-1", "type": "lamp", "x": 26, "y": -5.2},
+    {"id": "lamp-1", "type": "lamp", "x": 18, "y": -5.2},
     {"id": "lamp-2", "type": "lamp", "x": 44, "y": -5.2},
     {"id": "lamp-3", "type": "lamp", "x": -28, "y": 5.2},
     {"id": "lamp-4", "type": "lamp", "x": -44, "y": 5.2},