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/world/tiles/tilemap.rs

@@ -33,16 +33,18 @@ use rustc_hash::FxHashMap;
 
 use super::chunk_data::ChunkData;
 use crate::constants::ITILE_SIZE;
+use crate::entities::item::drop_table::DropTable;
 use crate::world::chunks::{world_to_chunk, CHUNK_SIZE, Z_ABOVE, Z_BELOW};
 
 /// Packed floor tile data for efficient storage. ~35 bytes vs 76 bytes tuple.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Debug)]
 pub struct FloorTileData {
     pub id: u8,
     /// bit0=can_stand_in, bit1=can_stand_on, bit2=visibly_transparent
     pub flags: u8,
     pub astar_weight: u8,
     pub visible_range: [u32; 8],
+    pub drop_table: DropTable,
 }
 
 impl Default for FloorTileData {
@@ -52,6 +54,7 @@ impl Default for FloorTileData {
             flags: 0b001,
             astar_weight: 0,
             visible_range: [0; 8],
+            drop_table: DropTable::default(),
         }
     }
 }
@@ -64,6 +67,7 @@ impl FloorTileData {
         visibly_transparent: bool,
         astar_weight: u8,
         visible_range: [u32; 8],
+        drop_table: DropTable,
     ) -> Self {
         let mut flags = 0u8;
         if can_stand_in {
@@ -80,6 +84,7 @@ impl FloorTileData {
             flags,
             astar_weight,
             visible_range,
+            drop_table,
         }
     }
 
@@ -214,6 +219,26 @@ impl TileMap {
         self.floor_tiles.insert(pos, tile);
     }
 
+    pub fn dig_floor(&mut self, pos: &IVec3) -> Option<FloorTileData> {
+        let removed = self.remove_floor(pos);
+        if removed.is_some() {
+            let air = crate::config::TileRegistry::global().floor("air");
+            self.insert_floor(
+                *pos,
+                FloorTileData::new(
+                    air.id,
+                    air.can_stand_in,
+                    air.can_stand_on,
+                    air.transparent,
+                    air.astar_weight,
+                    [0; 8],
+                    DropTable::default(),
+                ),
+            );
+        }
+        removed
+    }
+
     #[inline]
     pub fn insert_fixture(&mut self, pos: IVec3, tile: FixtureTileData) {
         let chunk_pos = world_to_chunk(pos);