perf: SmallVec inline storage, TOML drop tables, seeded deterministic RNG

- SmallVec<[DropEntry; 2]> replaces Vec in DropTable: zero heap allocation for
  all current tiles (0 or 1 entries), spills to heap only at 3+
- DropEntry fields packed to u8 (chance_pct, min_count, max_count): ~12 bytes ->
  4 bytes; derives Copy so no extra clones
- Replaced SystemTime pseudo-RNG with dig_rng(pos): PCG-style hash of world SEED
  + tile position. Deterministic per world, same dig = same roll every time
- TOML-driven drop tables: assets/drop_tables.toml (grass=5%/1-2, rock=10%/1,
  dirt/air=none). TOML parsed at startup into DropTableRegistry resource
- OnceLock global map for async terrain generation tasks to access drop tables
  without Bevy resource borrowing
- terrain.rs: DropTableRegistry::global_get("tile") replaces hardcoded drop_table_for

assets/drop_tables.toml

@@ -0,0 +1,15 @@
+[grass]
+drops = [
+    { prefab = "Coin", chance_pct = 5, min_count = 1, max_count = 2 },
+]
+
+[dirt]
+drops = []
+
+[rock]
+drops = [
+    { prefab = "Coin", chance_pct = 10, min_count = 1, max_max = 1 },
+]
+
+[air]
+drops = []