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/world/feature_scheduler.go

@@ -36,6 +36,12 @@ func (r *decorationRegion) replayScheduledOres(seed int64, targetX, targetZ int3
 		if err := r.placeScheduledGeodes(seed); err != nil {
 			return fmt.Errorf("world: replay source geodes (%d,%d): %w", source.X, source.Z, err)
 		}
+		// Stage 3 runs before the ores on purpose: the rooms' cave_air pockets
+		// are what vanilla's ore ellipsoids roll their air-exposure discards
+		// against.
+		if err := r.placeScheduledMonsterRooms(seed); err != nil {
+			return fmt.Errorf("world: replay source monster rooms (%d,%d): %w", source.X, source.Z, err)
+		}
 		if err := r.placeScheduledUndergroundOresStage(seed); err != nil {
 			return fmt.Errorf("world: replay source underground ores (%d,%d): %w", source.X, source.Z, err)
 		}