F2: continuous road grade — along-segment elevation + joint smoothing (D-021)

Road elevation is now sampled at the point on the road segment nearest the
column being carved, instead of at the segment's midpoint. The old behaviour
gave every column near a segment that segment's single midpoint height, so each
stretch of road was one flat plank and consecutive planks stepped like a
staircase wherever the road crossed a gradient.

Elevation now varies continuously along the segment, and because neighbouring
segments share an end point the height matches exactly at the joins. A new
ROAD_GRADE_SMOOTHING constant dials between holding a straight grade
(cut-and-fill) and hugging the land, per D-021.

Untouched: density field, Marching Cubes, chunk dimensions, the .dat contract,
and the 2D A* road network itself. Only how existing paths are carved into 3D.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Core/Scripts/Constants.cs

@@ -23,5 +23,23 @@ namespace IslaApocalypse.Core
         // Colour only. This does NOT affect terrain shape, collision, or what
         // block is actually stored in a voxel.
         public const float BLEND_BAND_METERS = 2.0f;
+
+        // --- ROAD GRADE TUNING ---------------------------------------------
+        // How much a road "holds a grade" instead of following every bump in
+        // the ground underneath it. Roads exist to be DRIVEN on, so this is a
+        // driveability setting, not a cosmetic one.
+        //
+        //   0.0 = hugs the land: the roadbed follows the terrain exactly.
+        //         Never steps, but inherits every lump the ground has.
+        //   1.0 = holds a grade (default): each stretch of road is a straight
+        //         ramp between its two path points, cutting through small
+        //         bumps and filling small dips. Smoothest to drive.
+        //
+        // Values in between mix the two. Raise toward 1.0 for a more
+        // engineered road, lower toward 0.0 for a more rustic one.
+        //
+        // NOTE: this affects road ELEVATION (terrain shape under the road),
+        // not colour. Tune it properly once vehicles exist to drive on it.
+        public const float ROAD_GRADE_SMOOTHING = 1.0f;
     }
 }