v0.8.0: map roads/sidewalks, per-vertex depth bias, retail camera controls

- ra3.map: capture the object RoadType; ra3.models pairs the map's
  Start/End road objects and drapes a flat textured ribbon along each
  segment (tessellated to the relief).
- Ground decals (roads/sidewalks) get a per-vertex screen-space depth
  offset (scene::vertex.bias) applied in the object shader and the
  software rasteriser, so they no longer z-fight or clip into the
  terrain at top-down angles.
- Tactical view aligned with retail (SAGE LookAtTranslator): middle-drag
  orbits yaw/pitch, a middle click resets angle/pitch/zoom, arrow keys
  pan, wheel zooms within the retail zoom range; WASD pan removed and
  left-drag no longer rotates.
- ui_event gains middle/right/released buttons; Vulkan + SDL map them.
- docs: roads, depth bias and camera controls.

shaders/object.vert

@@ -13,6 +13,7 @@ layout(location = 0) in vec3 in_pos;
 layout(location = 1) in vec3 in_normal;
 layout(location = 2) in vec2 in_uv;
 layout(location = 3) in float in_layer;
+layout(location = 4) in float in_bias;
 
 layout(push_constant) uniform Push {
     vec4 cam_pos; // xyz = eye position
@@ -36,7 +37,7 @@ void main() {
     float c = dot(r, pc.up.xyz);
     float th = pc.right.w;
     float th_aspect = pc.up.w;
-    float depth = clamp((a - NEAR) / (FAR - NEAR), 0.0, 1.0);
+    float depth = clamp((a - in_bias - NEAR) / (FAR - NEAR), 0.0, 1.0);
     gl_Position = vec4(b / th_aspect, -c / th, depth * a, a);
     out_uv = in_uv;
     out_normal = in_normal;