world: replay stage-3 monster rooms

Ports MonsterRoomFeature from bytecode with vanilla draw order and loop
structure: shell validation (solid floor and ceiling, one to five open
side columns), interior cave_air carving, cobblestone walls with the
mossy three-in-four floor roll drawn only where vanilla draws it, chest
placement faced away from its single opaque neighbour with the loot-seed
long consumed even though block entities are not modelled yet, and the
spawner's one-draw mob pick. Writes go through Feature.safeSetBlock
semantics against #minecraft:features_cannot_replace, captured verbatim
from the jar into the embedded datapack data.

The pass slots between geodes and the ores so the rooms' cave_air
pockets exist before ore ellipsoids roll their air-exposure discards.
The committed fixture's four chunks happen to contain no dungeons, so
measured parity holds at 98.028%, but the generator's output can differ
elsewhere: generatorVersion bumps to 22.

internal/worldgen/features_cannot_replace.go

@@ -0,0 +1,26 @@
+package worldgen
+
+import (
+	_ "embed"
+	"encoding/json"
+)
+
+// features_cannot_replace.json is #minecraft:features_cannot_replace, captured
+// verbatim from the 26.1.2 server jar. Features guard their writes through
+// Feature.safeSetBlock with this tag: an existing state in the tag stays, an
+// existing state outside it may be replaced.
+//
+//go:embed data/tag/features_cannot_replace.json
+var featuresCannotReplaceJSON []byte
+
+// FeaturesCannotReplace returns the block names in
+// #minecraft:features_cannot_replace.
+func FeaturesCannotReplace() ([]string, error) {
+	var doc struct {
+		Values []string `json:"values"`
+	}
+	if err := json.Unmarshal(featuresCannotReplaceJSON, &doc); err != nil {
+		return nil, err
+	}
+	return doc.Values, nil
+}