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/climate_sampler.go

@@ -0,0 +1,37 @@
+package worldgen
+
+// This file samples the climate density functions into a TargetPoint for the
+// biome finder. The climate router keys are 2D (flat_cache + y_scale=0) except
+// depth, which is 3D. For surface biome selection we fix depth to 0.0, matching
+// the depth=0 (surface) entries of the biome parameter table; underground and
+// cave biomes use depth=1.0 / non-zero offset and are a later milestone.
+
+// SampleColumn evaluates the six climate parameters at block (wx, wz) using od
+// and returns the TargetPoint for surface biome lookup. seaLevelY is the Y at
+// which to sample the 2D climate noises (callers pass the world sea level).
+func SampleColumn(od *OverworldDensity, seaLevelY int, wx, wz int) TargetPoint {
+	ctx := FunctionContext{X: float64(wx), Y: float64(seaLevelY), Z: float64(wz)}
+
+	temp := computeOrZero(od.Temperature, ctx)
+	humid := computeOrZero(od.Humidity, ctx)
+	cont := computeOrZero(od.Continentalness, ctx)
+	ero := computeOrZero(od.Erosion, ctx)
+	weird := computeOrZero(od.Weirdness, ctx)
+
+	// Surface layer: depth axis is fixed at 0.0 so only the depth=0 (surface)
+	// biome parameter entries match. The real 3D depth is consulted in the
+	// per-cell milestone.
+	const surfaceDepth = 0.0
+
+	return NewTargetPoint(temp, humid, cont, ero, weird, surfaceDepth)
+}
+
+// computeOrZero evaluates df at ctx, returning 0 when df is nil (a climate key
+// absent from the router). This keeps sampling robust without special-casing
+// each axis at the call site.
+func computeOrZero(df DensityFunction, ctx FunctionContext) float64 {
+	if df == nil {
+		return 0
+	}
+	return df.Compute(ctx)
+}