feat(core): add temperature and humidity climate sampling

core/include/vf/Climate.h

@@ -0,0 +1,74 @@
+#pragma once
+
+#include <cstdint>
+#include <unordered_map>
+#include <vector>
+#include "World.h"
+#include "Math.h"
+#include "BiomeGen.h"
+#include "Weather.h"
+
+namespace vf {
+
+struct ClimateChange {
+    Vec3i pos;
+    Block from;
+    Block to;
+};
+
+struct ClimateConfig {
+    int seaLevel = 26;
+    // Temperature drop per block of elevation above sea level.
+    float heightLapse = 0.006f;
+    // Temperature drop per block of distance from the z=0 "equator".
+    float latitudeLapse = 0.0009f;
+    // Cold enough to lay snow and freeze exposed water.
+    float freezeTemperature = 0.15f;
+    // Warm enough to thaw ice and melt snow (hysteresis above freezing).
+    float meltTemperature = 0.5f;
+    // Milder threshold at which water freezes if it already touches ice, so a
+    // frozen patch creeps outward across a surface.
+    float frostSpreadTemperature = 0.30f;
+    int maxSnowDepth = 8;
+    // Ticks a block of ice must survive in the cold before it compacts to
+    // packed ice.
+    int packThreshold = 4;
+};
+
+// Deterministic per-region climate simulation layered on a BiomeGen. Column
+// temperature is the biome sample adjusted for elevation and latitude. During
+// snowfall it accumulates SnowLayer depth on exposed surfaces; exposed water
+// freezes to Ice when cold; Ice and snow thaw when warm or beside a heat source
+// (lava, torches, glowstone). tick scans the region in a fixed order and emits
+// the resulting block changes without mutating the world; apply commits them.
+class Climate {
+public:
+    Climate(const BiomeGen& biomes, const ClimateConfig& config = {});
+
+    float temperatureAt(int gx, int gy, int gz) const;
+    float humidityAt(int gx, int gz) const;
+    bool freezingAt(int gx, int gy, int gz) const;
+    bool thawingAt(int gx, int gy, int gz) const;
+
+    std::vector<ClimateChange> tick(World& world, const Vec3i& origin, const Vec3i& dims,
+                                  Precipitation precip);
+
+    static void apply(World& world, const std::vector<ClimateChange>& changes);
+
+    int snowDepthAt(const Vec3i& pos) const;
+    int iceAgeAt(const Vec3i& pos) const;
+    std::size_t trackedSnowColumns() const { return snowDepth.size(); }
+
+    const ClimateConfig& settings() const { return config; }
+
+private:
+    bool heatSourceNear(const World& world, const Vec3i& pos) const;
+    bool iceNear(const World& world, const Vec3i& pos) const;
+
+    const BiomeGen& biomes;
+    ClimateConfig config;
+    std::unordered_map<Vec3i, int, Vec3iHash> snowDepth;
+    std::unordered_map<Vec3i, int, Vec3iHash> iceAge;
+};
+
+} // namespace vf