fix: preserve biome priority boundaries

modules/world-worldgen/src/biome_registry.zig

@@ -11,6 +11,9 @@ pub const TreeType = tree_registry.TreeType;
 /// Minimum sum threshold for biome blend calculation to avoid division by near-zero values
 pub const BLEND_EPSILON: f32 = 0.0001;
 
+/// Normalized elevation produced by computeClimateParams at sea level.
+pub const NORMALIZED_SEA_LEVEL: f32 = 0.30;
+
 /// Represents a range of values for biome parameter matching
 pub const Range = struct {
     min: f32,
@@ -387,6 +390,7 @@ pub const BIOME_REGISTRY: []const BiomeDefinition = &.{
         .humidity = Range.any(),
         .elevation = .{ .min = 0.28, .max = 0.38 },
         .continentalness = .{ .min = 0.35, .max = 0.42 }, // NARROW beach band
+        .max_height = 70,
         .max_slope = 2,
         .priority = 10,
         .surface = .{ .top = .sand, .filler = .sand, .depth_range = 2 },
@@ -399,6 +403,7 @@ pub const BIOME_REGISTRY: []const BiomeDefinition = &.{
         .humidity = Range.any(),
         .elevation = .{ .min = 0.28, .max = 0.45 },
         .continentalness = .{ .min = 0.35, .max = 0.45 },
+        .max_height = 82,
         .max_slope = 8,
         .priority = 11,
         .surface = .{ .top = .stone, .filler = .gravel, .depth_range = 2 },
@@ -413,6 +418,7 @@ pub const BIOME_REGISTRY: []const BiomeDefinition = &.{
         .humidity = Range.any(),
         .elevation = .{ .min = 0.28, .max = 0.38 },
         .continentalness = .{ .min = 0.35, .max = 0.42 },
+        .max_height = 70,
         .max_slope = 2,
         .priority = 12,
         .surface = .{ .top = .snow_block, .filler = .sand, .depth_range = 2 },

modules/world-worldgen/src/biome_selector.zig

@@ -10,6 +10,9 @@ const StructuralParams = registry.StructuralParams;
 const BIOME_REGISTRY = registry.BIOME_REGISTRY;
 const BIOME_POINTS = registry.BIOME_POINTS;
 const BLEND_EPSILON = registry.BLEND_EPSILON;
+const NORMALIZED_SEA_LEVEL = registry.NORMALIZED_SEA_LEVEL;
+
+const OCEAN_CONTINENTALNESS_MAX: f32 = 0.35;
 
 // ============================================================================
 // Voronoi Biome Selection (Issue #106)
@@ -107,6 +110,7 @@ pub fn selectBiome(params: ClimateParams) BiomeId {
     var best_biome: BiomeId = .plains; // Default fallback
 
     for (BIOME_REGISTRY) |biome| {
+        if (isOceanBiome(biome.id) and !isOceanClimate(params)) continue;
         const s = biome.scoreClimate(params);
         if (s > best_score) {
             best_score = s;
@@ -129,6 +133,7 @@ pub fn selectBiomeMultiParam(climate: ClimateParams, structural: StructuralParam
     var best_biome: BiomeId = .plains;
 
     for (BIOME_REGISTRY) |biome| {
+        if (isOceanBiome(biome.id) and !isOceanStructure(params, structural)) continue;
         if (!biome.meetsStructuralConstraints(structural.height, structural.slope, structural.continentalness, structural.ridge_mask)) continue;
 
         const s = biome.scoreClimate(params);
@@ -141,6 +146,21 @@ pub fn selectBiomeMultiParam(climate: ClimateParams, structural: StructuralParam
     return best_biome;
 }
 
+fn isOceanStructure(climate: ClimateParams, structural: StructuralParams) bool {
+    return structural.continentalness < OCEAN_CONTINENTALNESS_MAX and climate.elevation <= NORMALIZED_SEA_LEVEL;
+}
+
+fn isOceanClimate(climate: ClimateParams) bool {
+    return climate.continentalness < OCEAN_CONTINENTALNESS_MAX and climate.elevation <= NORMALIZED_SEA_LEVEL;
+}
+
+fn isOceanBiome(biome: BiomeId) bool {
+    return switch (biome) {
+        .deep_ocean, .ocean, .frozen_ocean, .cold_ocean, .warm_ocean => true,
+        else => false,
+    };
+}
+
 /// Select biome with river override
 pub fn selectBiomeWithRiver(params: ClimateParams, river_mask: f32) BiomeId {
     // River biome takes priority when river mask is active
@@ -161,18 +181,18 @@ pub fn computeClimateParams(
     sea_level: i32,
     max_height: i32,
 ) ClimateParams {
-    // Normalize elevation: 0 = below sea, 0.3 = sea level, 1.0 = max height
+    // Normalize elevation: 0 = below sea, NORMALIZED_SEA_LEVEL = sea level, 1.0 = max height
     // Use conditional to avoid integer overflow when height < sea_level
     const height_above_sea: i32 = if (height > sea_level) height - sea_level else 0;
     const elevation_range = max_height - sea_level;
     const elevation = if (elevation_range > 0)
-        0.3 + 0.7 * @as(f32, @floatFromInt(height_above_sea)) / @as(f32, @floatFromInt(elevation_range))
+        NORMALIZED_SEA_LEVEL + 0.7 * @as(f32, @floatFromInt(height_above_sea)) / @as(f32, @floatFromInt(elevation_range))
     else
-        0.3;
+        NORMALIZED_SEA_LEVEL;
 
-    // For underwater: scale 0-0.3
+    // For underwater: scale 0-NORMALIZED_SEA_LEVEL
     const final_elevation = if (height < sea_level)
-        0.3 * @as(f32, @floatFromInt(@max(0, height))) / @as(f32, @floatFromInt(sea_level))
+        NORMALIZED_SEA_LEVEL * @as(f32, @floatFromInt(@max(0, height))) / @as(f32, @floatFromInt(sea_level))
     else
         elevation;
 
@@ -206,6 +226,7 @@ pub fn selectBiomeBlended(params: ClimateParams) BiomeSelection {
     var second_biome: ?BiomeId = null;
 
     for (BIOME_REGISTRY) |biome| {
+        if (isOceanBiome(biome.id) and !isOceanClimate(params)) continue;
         const s = biome.scoreClimate(params);
         if (s > best_score) {
             second_score = best_score;

modules/world-worldgen/src/biome_selector_tests.zig

@@ -393,6 +393,114 @@ test "selectBiomeWithConstraints locks structural edge cases" {
     }));
 }
 
+test "selectBiomeWithConstraints preserves ocean sea-level boundary" {
+    const underwater_ocean = ClimateParams{
+        .temperature = 0.5,
+        .humidity = 0.5,
+        .elevation = 0.30,
+        .continentalness = 0.25,
+        .ruggedness = 0.1,
+    };
+    const inland_ocean_candidate = ClimateParams{
+        .temperature = 0.5,
+        .humidity = 0.5,
+        .elevation = 0.31,
+        .continentalness = 0.25,
+        .ruggedness = 0.1,
+    };
+    const structural = StructuralParams{
+        .height = 64,
+        .slope = 1,
+        .continentalness = 0.25,
+        .ridge_mask = 0.1,
+    };
+
+    try testing.expectEqual(BiomeId.ocean, selectBiomeWithConstraints(underwater_ocean, structural));
+    try testing.expect(selectBiomeWithConstraints(inland_ocean_candidate, structural) != .ocean);
+    try testing.expect(selectBiomeWithConstraints(inland_ocean_candidate, structural) != .deep_ocean);
+    try testing.expect(selectBiomeWithConstraints(inland_ocean_candidate, structural) != .frozen_ocean);
+    try testing.expect(selectBiomeWithConstraints(inland_ocean_candidate, structural) != .cold_ocean);
+    try testing.expect(selectBiomeWithConstraints(inland_ocean_candidate, structural) != .warm_ocean);
+}
+
+test "selectBiomeWithConstraints preserves coast and inland water boundaries" {
+    const coast_climate = ClimateParams{
+        .temperature = 0.6,
+        .humidity = 0.5,
+        .elevation = 0.3,
+        .continentalness = 0.35,
+        .ruggedness = 0.1,
+    };
+    const coast_structural = StructuralParams{
+        .height = 64,
+        .slope = 1,
+        .continentalness = 0.35,
+        .ridge_mask = 0.1,
+    };
+    const steep_coast = StructuralParams{
+        .height = 64,
+        .slope = 3,
+        .continentalness = 0.35,
+        .ridge_mask = 0.1,
+    };
+    const high_coast = StructuralParams{
+        .height = 71,
+        .slope = 1,
+        .continentalness = 0.35,
+        .ridge_mask = 0.1,
+    };
+    const inland_water = ClimateParams{
+        .temperature = 0.6,
+        .humidity = 0.5,
+        .elevation = 0.2,
+        .continentalness = 0.6,
+        .ruggedness = 0.1,
+    };
+
+    try testing.expectEqual(BiomeId.beach, selectBiomeWithConstraints(coast_climate, coast_structural));
+    try testing.expect(selectBiomeWithConstraints(coast_climate, steep_coast) != .beach);
+    try testing.expect(selectBiomeWithConstraints(coast_climate, high_coast) != .beach);
+    try testing.expect(selectBiomeWithConstraints(inland_water, .{
+        .height = 50,
+        .slope = 1,
+        .continentalness = 0.6,
+        .ridge_mask = 0.1,
+    }) != .beach);
+}
+
+test "selectBiomeWithConstraintsAndRiver preserves river thresholds" {
+    const temperate = ClimateParams{
+        .temperature = 0.21,
+        .humidity = 0.7,
+        .elevation = 0.3,
+        .continentalness = 0.6,
+        .ruggedness = 0.2,
+    };
+    const frozen = ClimateParams{
+        .temperature = 0.20,
+        .humidity = 0.7,
+        .elevation = 0.3,
+        .continentalness = 0.6,
+        .ruggedness = 0.2,
+    };
+    const structural = StructuralParams{
+        .height = 119,
+        .slope = 2,
+        .continentalness = 0.6,
+        .ridge_mask = 0.1,
+    };
+
+    try testing.expect(selectBiomeWithConstraintsAndRiver(temperate, structural, 0.5) != .river);
+    try testing.expectEqual(BiomeId.river, selectBiomeWithConstraintsAndRiver(temperate, structural, 0.5001));
+    try testing.expectEqual(BiomeId.frozen_river, selectBiomeWithConstraintsAndRiver(frozen, structural, 0.5001));
+    try testing.expect(selectBiomeWithConstraintsAndRiver(temperate, .{
+        .height = 120,
+        .slope = 2,
+        .continentalness = 0.6,
+        .ridge_mask = 0.1,
+    }, 0.5001) != .river);
+}
+
 test "selectBiomeWithConstraints documents ridge mask baseline behavior" {
     const climate = ClimateParams{
         .temperature = 0.4,
@@ -547,6 +655,22 @@ test "selectBiomeSimple returns ocean for low continentalness" {
     try testing.expect(biome == .deep_ocean or biome == .ocean);
 }
 
+test "selectBiomeSimple does not return ocean above sea level" {
+    const params = ClimateParams{
+        .temperature = 0.5,
+        .humidity = 0.5,
+        .elevation = 0.31,
+        .continentalness = 0.15,
+        .ruggedness = 0.3,
+    };
+    const biome = selectBiomeSimple(params);
+    try testing.expect(biome != .deep_ocean);
+    try testing.expect(biome != .ocean);
+    try testing.expect(biome != .frozen_ocean);
+    try testing.expect(biome != .cold_ocean);
+    try testing.expect(biome != .warm_ocean);
+}
+
 test "selectBiomeSimple returns deep_ocean for very low continentalness" {
     const params = ClimateParams{
         .temperature = 0.5,