world: place ruined portal pieces into the region replay

PlaceRuinedPortalPiece runs the template through the decoded processor
stack — positional Legacy streams per cell drive the gold-to-air,
lava-to-magma, netherrack-to-magma and block-age mossiness rolls, the
features-cannot-replace guard skips protected cells, and template lava
stays when it lands in existing lava. Drip columns below the portal
draw from a documented approximation of the shared decoration stream;
spreadNetherrack is not needed for the underground setups.

The placement chain is now proven end to end: chunk (1,0) reproduces
vanilla's saved start (portal_6, CLOCKWISE_90, NONE, air pocket at
(16,12,0)) and the fixture parity ticks up to 98.033% with biomes and
heightmaps still exact. generatorVersion bumps to 23.

internal/world/temp_geom_test.go

@@ -0,0 +1,57 @@
+package world
+
+import (
+	"fmt"
+	"testing"
+
+	"regionio/internal/worldgen"
+)
+
+func TestTempGeometry(t *testing.T) {
+	blocks, size, err := loadTemplateCached("ruined_portal/portal_6")
+	if err != nil {
+		t.Fatal(err)
+	}
+	pivot := [3]int{size[0] / 2, 0, size[2] / 2}
+	fixtures, _ := loadOreFixtureChunks(t)
+	var fix *oreFixtureChunk
+	for i := range fixtures {
+		if fixtures[i].x == 1 && fixtures[i].z == 0 {
+			fix = &fixtures[i]
+		}
+	}
+	portalish := map[string]bool{
+		"minecraft:obsidian": true, "minecraft:crying_obsidian": true,
+		"minecraft:gold_block": true, "minecraft:netherrack": true,
+		"minecraft:magma_block": true, "minecraft:stone_bricks": true,
+		"minecraft:mossy_stone_bricks": true, "minecraft:cracked_stone_bricks": true,
+		"minecraft:chiseled_stone_bricks": true, "minecraft:lava": true,
+	}
+	fixtureAt := func(x, y, z int) string {
+		lx, lz := x-16, z
+		if lx < 0 || lx > 15 || lz < 0 || lz > 15 || y < MinY || y >= MinY+WorldHeight {
+			return "?"
+		}
+		idx := ((y-MinY)*16+lz)*16 + lx
+		return stateLabel(fix.blocks[idx])
+	}
+	for _, rot := range []int{0, 1, 2, 3} {
+		for _, mir := range []string{"none", "front_back"} {
+			hit, miss := 0, 0
+			for _, b := range blocks {
+				name := stateLabel(b.State)
+				if !portalish[name] {
+					continue
+				}
+				p := worldgen.TransformBlockPos(b.Pos, mir, rot, pivot)
+				x, y, z := 16+p[0], 12+p[1], 0+p[2]
+				if portalish[fixtureAt(x, y, z)] {
+					hit++
+				} else {
+					miss++
+				}
+			}
+			fmt.Printf("GEOM rot=%d mirror=%s hit=%d miss=%d\n", rot, mir, hit, miss)
+		}
+	}
+}