fix: dedupe float samples in the monotonicity assertion (terrain-water task 05)

The coarse and fine sampling loops both touched h=0.10 and the two
doubles round to the same float32, so the strict check compared the
identity value against itself and threw on every curve-on run. Only
strictly increasing float samples are compared now. The curve itself
was never non-monotonic.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Tools/Scripts/HeightCurve.cs

@@ -85,20 +85,28 @@ public static class HeightCurve
 	/// </summary>
 	public static void AssertMonotonic()
 	{
-		float prev = Apply(-7f);
+		float prevH = -7f;
-		// Coarse below the identity region, fine through every knot, out past the tail.
+		float prev = Apply(prevH);
-		for (double h = -7.0 + 0.01; h < 0.10; h += 0.01) prev = Step(prev, (float)h);
-		for (double h = 0.10; h <= 2.0; h += 0.0001) prev = Step(prev, (float)h);
-		for (double h = 2.0; h <= 8.0; h += 0.05) prev = Step(prev, (float)h);
-		GD.Print("[HeightCurve] Monotonicity assertion passed (v" + VERSION + ").");
-	}
 
-	private static float Step(float prev, float h)
+		// Successive double samples can round to the SAME float32 (the two loops
+		// meeting at 0.10 did exactly that and tripped the strict check against
+		// itself) — so only strictly increasing float samples are compared.
+		void Check(double hd)
 		{
+			float h = (float)hd;
+			if (h <= prevH) return;
 			float v = Apply(h);
 			if (v <= prev)
 				throw new System.InvalidOperationException(
 					$"[HeightCurve] MONOTONICITY VIOLATION at h={h}: {v} <= {prev}. Refusing to generate.");
-		return v;
+			prev = v;
+			prevH = h;
+		}
+
+		// Coarse below the identity region, fine through every knot, out past the tail.
+		for (double h = -7.0 + 0.01; h < 0.10; h += 0.01) Check(h);
+		for (double h = 0.10; h <= 2.0; h += 0.0001) Check(h);
+		for (double h = 2.05; h <= 8.0; h += 0.05) Check(h);
+		GD.Print("[HeightCurve] Monotonicity assertion passed (v" + VERSION + ").");
 	}
 }