feat(config): moderate spawn tuning for Alex's Mobs, Fowl Play, Critters

Every visible entity multiplies the client render cost (vanilla, EMF/FA,
GeckoLib, ETF, dynamic lights) and the server tick. These cuts target only the
species that are heavy by weight x group size. Rare mobs keep their defaults.

- Alex's Mobs: halve 9 common weights (gazelle and capuchin herds, straddler,
  rocky roller, underminer, elephant, blobfish, mudskipper, caiman).
- Fowl Play: lower duck/goose/gull/sparrow weights and max group sizes
  (upstream #214, ~400 ducks on rivers, is still open in 1.2.3).
- Critters and Companions: about 40% off sea bunnies and the forest/lush bug
  rules. It does have a spawn config; configVersion 2 skips its v1->v2
  migration.

All three are partial files. NeoForge (TOML) and YACL 3.8.2 (JSON5) keep
defaults for missing keys. Shipped enforced from config/ because the server
reads them from this repo. Details and the B14 measurement plan:
analysis/client-perf/SPAWN-TUNING.md.

config/crittersandcompanions-common.json5

@@ -0,0 +1,51 @@
+// Onion MC spawn tuning (analysis/client-perf/SPAWN-TUNING.md). Partial file: YACL keeps
+// defaults for missing keys. Each listed rule list REPLACES that mob's defaults, so every
+// biome rule of a changed mob is listed. configVersion 2 skips the v1->v2 migration.
+{
+  "configVersion": 2,
+  "spawning": {
+    "leafInsect": [
+      {"biomeSpec": "#c:is_jungle", "weight": 8, "min": 1, "max": 1},
+      {"biomeSpec": "#c:is_forest", "weight": 4, "min": 1, "max": 1}
+    ],
+    "seaBunny": [
+      {"biomeSpec": "minecraft:ocean", "weight": 10, "min": 1, "max": 2},
+      {"biomeSpec": "minecraft:deep_ocean", "weight": 10, "min": 1, "max": 2},
+      {"biomeSpec": "minecraft:warm_ocean", "weight": 20, "min": 1, "max": 3},
+      {"biomeSpec": "minecraft:lukewarm_ocean", "weight": 10, "min": 1, "max": 3},
+      {"biomeSpec": "minecraft:deep_lukewarm_ocean", "weight": 10, "min": 1, "max": 3}
+    ],
+    "dragonfly": [
+      {"biomeSpec": "#c:is_river", "weight": 4, "min": 1, "max": 1},
+      {"biomeSpec": "#c:is_swamp", "weight": 5, "min": 1, "max": 1},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 1}
+    ],
+    "ladybug": [
+      {"biomeSpec": "#c:is_forest", "weight": 4, "min": 1, "max": 3},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_floral", "weight": 7, "min": 1, "max": 2}
+    ],
+    "stagBeetle": [
+      {"biomeSpec": "#c:is_forest", "weight": 3, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 2}
+    ],
+    "rolyPoly": [
+      {"biomeSpec": "#c:is_forest", "weight": 3, "min": 1, "max": 3},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_swamp", "weight": 6, "min": 1, "max": 2}
+    ],
+    "snail": [
+      {"biomeSpec": "#c:is_forest", "weight": 3, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_lush", "weight": 5, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_swamp", "weight": 6, "min": 1, "max": 2}
+    ],
+    "stickBug": [
+      {"biomeSpec": "#c:is_forest", "weight": 3, "min": 1, "max": 2},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 2}
+    ],
+    "weevil": [
+      {"biomeSpec": "#c:is_forest", "weight": 3, "min": 1, "max": 3},
+      {"biomeSpec": "#c:is_lush", "weight": 6, "min": 1, "max": 2}
+    ]
+  }
+}