world: ruined portal stub matches vanilla's saved start bit-for-bit

Three fixes close the loop against the captured region file: the giant
roll precedes the template draw, surfaceY is the topmost non-air block
(getHeight already returns one past it), and structure biome refs
resolve through has_structure/ tags to the extracted tag files. The
weighted variant pick walks a fresh per-attempt GenerationContext
stream while failed entries drop from the pick stream without reseeding.

TestRuinedPortalStubMatchesVanillaStart pins chunk (1,0) to vanilla's
own saved start: portal_6, CLOCKWISE_90, mirror NONE, air pocket,
template position (16,12,0) — and asserts the neighbouring fixture
chunks stay portal-free.

internal/world/ruined_portal.go

@@ -199,9 +199,12 @@ func ruinedPortalVariantPoint(od *worldgen.OverworldDensity, sets *worldgen.Stru
 
 	airPocket := sampleProbability(random, setup.AirPocketProbability)
 
-	templateName := ruinedPortalTemplates[int(random.NextIntN(int32(len(ruinedPortalTemplates))))]
+	// Giant roll comes FIRST; only a normal roll draws the template index.
+	templateName := ""
 	if random.NextFloat() < 0.05 {
 		templateName = ruinedPortalGiants[int(random.NextIntN(int32(len(ruinedPortalGiants))))]
+	} else {
+		templateName = ruinedPortalTemplates[int(random.NextIntN(int32(len(ruinedPortalTemplates))))]
 	}
 	_, size, err := loadTemplateCached(templateName)
 	if err != nil {

internal/world/ruined_portal_test.go

@@ -0,0 +1,47 @@
+package world
+
+import (
+	"testing"
+
+	"regionio/internal/worldgen"
+)
+
+// TestRuinedPortalStubMatchesVanillaStart pins the whole placement chain —
+// grid claim, weighted variant pick, setup/template/rotation/mirror draws,
+// findSuitableY against pre-carve heights, and the 3D-biome filter — to the
+// start vanilla itself saved into the captured chunk (1,0): template
+// ruined_portal/portal_6, CLOCKWISE_90, mirror NONE, air pocket on,
+// template position (16,12,0).
+func TestRuinedPortalStubMatchesVanillaStart(t *testing.T) {
+	od, err := worldgen.LoadOverworldFinalDensity(12345)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sets, err := worldgen.LoadStructureSets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	stub, err := RuinedPortalGenerationPoint(od, sets, 12345, 1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stub == nil {
+		t.Fatal("no stub, but vanilla's captured start lives here")
+	}
+	if stub.Template != "ruined_portal/portal_6" ||
+		stub.Rotation != 1 || stub.Mirror != "none" ||
+		!stub.AirPocket || stub.X != 16 || stub.Y != 12 || stub.Z != 0 {
+		t.Fatalf("stub %+v does not match vanilla's saved start", *stub)
+	}
+	// The neighbouring fixture chunks must stay portal-free: vanilla stored
+	// no other ruined_portal starts nearby.
+	for _, c := range [][2]int32{{0, 0}, {0, 1}, {-1, -1}} {
+		other, err := RuinedPortalGenerationPoint(od, sets, 12345, c[0], c[1])
+		if err != nil {
+			t.Fatal(err)
+		}
+		if other != nil {
+			t.Fatalf("unexpected extra stub at (%d,%d): %+v", c[0], c[1], *other)
+		}
+	}
+}

internal/world/temp_verify_test.go

@@ -0,0 +1,31 @@
+package world
+
+import (
+	"testing"
+
+	"regionio/internal/worldgen"
+)
+
+func TestTempRPVerify(t *testing.T) {
+	od, err := worldgen.LoadOverworldFinalDensity(12345)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sets, err := worldgen.LoadStructureSets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	stub, err := RuinedPortalGenerationPoint(od, sets, 12345, 1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stub == nil {
+		t.Fatal("no stub, but vanilla has one here")
+	}
+	t.Logf("stub=(%d,%d,%d) tmpl=%s rot=%d mir=%s air=%v",
+		stub.X, stub.Y, stub.Z, stub.Template[14:], stub.Rotation, stub.Mirror, stub.AirPocket)
+	if stub.Template != "ruined_portal/portal_6" || stub.Rotation != 1 || stub.Mirror != "none" ||
+		!stub.AirPocket || stub.Y != 12 || stub.X != 16 || stub.Z != 0 {
+		t.Fatalf("mismatch vs vanilla start portal_6 CW90 NONE air TPY=12 at (16,y,0)")
+	}
+}

internal/worldgen/structures.go

@@ -270,11 +270,15 @@ func (s *StructureSets) StructuresInSet(set string) []*StructureDef {
 }
 
 // BiomesFor resolves a structure def's biome reference ("#tag" or a single
-// name) to concrete biome names with the minecraft: prefix.
+// name) to concrete biome names with the minecraft: prefix. Structure JSONs
+// reference tags through the has_structure/ namespace, but the extracted tag
+// files drop that prefix.
 func (s *StructureSets) BiomesFor(def *StructureDef) []string {
 	ref := def.Biomes
 	if strings.HasPrefix(ref, "#") {
-		return s.BiomeTags["minecraft:"+strings.TrimPrefix(ref[1:], "minecraft:")]
+		name := strings.TrimPrefix(ref[1:], "minecraft:")
+		name = strings.TrimPrefix(name, "has_structure/")
+		return s.BiomeTags["minecraft:"+name]
 	}
 	return []string{"minecraft:" + ref}
 }