refactor: extract generic digging system, drop tables, remove debug code

- New src/entities/item/drop_table.rs: DropEntry (chance, min/max count,
  pseudo-RNG roll) + DropTable wrapper. grass=5% coin 1-2, rock=10% coin 1,
  dirt/air=none.
- New src/entities/shared_systems/digging.rs: Digger component + dig_system.
  Any entity with Digger digs the tile below on its interval. Replaces
  rabbit_dig_system/rabbit_fall_debug_system/RabbitDigTimer/RabbitFallDebug.
- New TileMap::dig_floor: remove_floor + insert air. Used by dig_system.
- FloorTileData: added drop_table field. Lost Copy derive (Vec field).
  Updated all 6 terrain.rs call sites with per-tile drop tables.
- Pig: removed PigDropTimer + pig_drop_system. Drops were debug placeholder.
  TODO added for future loot-on-death/butcher system.
- Rabbit: removed all debug components/systems. Now uses Digger::new(5.0).
- main.rs: removed rabbit_dig_system/rabbit_fall_debug_system/pig_drop_system,
  added shared_systems::digging::dig_system.

src/main.rs

@@ -3,7 +3,7 @@ use bevy_rand::prelude::*;
 
 use crate::entities::{
     item::{initialize_item_rotation_state, item_tile_management_system, ItemRotationTimer},
-    livestock::pig::pig_drop_system,
+    shared_systems::digging::dig_system,
 };
 use crate::game::SpawnDelay;
 
@@ -45,10 +45,6 @@ fn main() {
         .insert_resource(ClearColor(Color::srgb(0., 0., 0.)))
         .add_plugins(world::WorldPlugin)
         .add_plugins(entities::pathfinding::PathfindingPlugin)
-        .add_systems(
-            FixedUpdate,
-            entities::livestock::rabbit::rabbit_fall_debug_system,
-        )
         .add_systems(
             Startup,
             (camera::spawn_panning_camera, cursor::setup_cursor),
@@ -63,10 +59,10 @@ fn main() {
                 entities::sentient::dorf::spawn_dorfs,
                 entities::livestock::pig::spawn_pigs,
                 entities::livestock::rabbit::spawn_rabbits,
-                entities::livestock::rabbit::rabbit_dig_system,
+                dig_system,
             ),
         )
-        .add_systems(Update, pig_drop_system)
+
         .insert_resource(ItemRotationTimer::default())
         .add_systems(Update, initialize_item_rotation_state)
         .add_systems(