Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

internal/worldgen/spline.go

@@ -0,0 +1,46 @@
+package worldgen
+
+// CubicSpline is Minecraft's cubic spline over a coordinate density function.
+// Each point's value is itself a density function (often a constant or a nested
+// spline), enabling multi-dimensional terrain shaping.
+type CubicSpline struct {
+	coordinate  DensityFunction
+	locations   []float32
+	values      []DensityFunction
+	derivatives []float32
+}
+
+func (s *CubicSpline) Compute(c FunctionContext) float64 {
+	t := float32(s.coordinate.Compute(c))
+	n := len(s.locations)
+
+	// Below the first point or above the last: linear extrapolation.
+	if t < s.locations[0] {
+		return float64(extend(s.values[0].Compute(c), s.derivatives[0], s.locations[0], t))
+	}
+	if t >= s.locations[n-1] {
+		return float64(extend(s.values[n-1].Compute(c), s.derivatives[n-1], s.locations[n-1], t))
+	}
+
+	// Find the segment [i, i+1] containing t.
+	i := 0
+	for i < n-1 && s.locations[i+1] <= t {
+		i++
+	}
+	loc0, loc1 := s.locations[i], s.locations[i+1]
+	d0, d1 := s.derivatives[i], s.derivatives[i+1]
+	p0 := float32(s.values[i].Compute(c))
+	p1 := float32(s.values[i+1].Compute(c))
+
+	k := (t - loc0) / (loc1 - loc0)
+	l := d0*(loc1-loc0) - (p1 - p0)
+	m := -d1*(loc1-loc0) + (p1 - p0)
+	return float64(lerp32(k, p0, p1) + k*(1-k)*lerp32(k, l, m))
+}
+
+// extend linearly extrapolates a point's value beyond the spline's range.
+func extend(value float64, derivative, loc, t float32) float32 {
+	return float32(value) + derivative*(t-loc)
+}
+
+func lerp32(t, a, b float32) float32 { return a + t*(b-a) }