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_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)
+		}
+	}
+}