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/data/tag/features_cannot_replace.json

@@ -0,0 +1,11 @@
+{
+  "values": [
+    "minecraft:bedrock",
+    "minecraft:spawner",
+    "minecraft:chest",
+    "minecraft:end_portal_frame",
+    "minecraft:reinforced_deepslate",
+    "minecraft:trial_spawner",
+    "minecraft:vault"
+  ]
+}