docs: update README for generic digging, drop tables, revised architecture

- Add generic digging + drop tables to implemented list
- Rabbit digging: now using Digger component, not debug code
- Building and construction: updated to reflect generic system in place
- Architecture: added digging to shared_systems, drop_table to tiles

README.md

@@ -14,13 +14,13 @@ src/
 ├── entities/          # Dorfs, pigs, rabbits — ambulatory entities
 │   ├── sentient/
 │   ├── livestock/
-│   └── shared_*/     # Ambulatory, pathfinding, occupancy systems
+│   └── shared_*/     # Ambulatory, pathfinding, occupancy, digging systems
 ├── game.rs            # ZIndex resource, frame timing
 ├── main.rs            # Bevy app bootstrap
 └── world/
     ├── chunks/        # ChunkMap, chunk loading/unloading, connectivity
     ├── generation/    # Terrain blobs, forestry, foliage (stub), fauna (stub)
-    └── tiles/         # TileMap, TileRegistry, rendering, visibility, tile change events
+    └── tiles/         # TileMap, TileRegistry, rendering, visibility, tile change events, drop tables
 ```
 
 ### Chunk Architecture
@@ -95,7 +95,12 @@ Mobile entities (dorfs, pigs, rabbits) are **not** in `chunk_entity_index`. They
 - [x] Render chunk dirty-tracking for incremental rebakes
 - [x] Stage 2 collision resolution (convoy, E/S yield, W/N right-of-way)
 - [x] TileChangedEvent + path invalidation on tile change
-- [x] Rabbit digging (debug) — removes floor tile, replaces with air, fires occlusion + path invalidation
+- [x] Generic tile digging — `Digger` component on any entity, `dig_system` handles
+  tile removal, item drops via drop tables, occlusion refresh, path invalidation
+- [x] Drop tables — tiles carry `DropTable` defining item drops on dig (grass=5% coin,
+  rock=10% coin, dirt/air=none). Pseudo-RNG per position until WyRand is accessible.
+- [x] Rabbit digging — `Digger::new(5.0)` on rabbits; removes floor tile, drops items,
+  fires occlusion + path invalidation
 
 ### Stubs (exist as empty functions, wired into schedules)
 - [ ] Foliage generation per chunk (`generate_chunk_foliage`)
@@ -106,7 +111,7 @@ Mobile entities (dorfs, pigs, rabbits) are **not** in `chunk_entity_index`. They
 - [ ] Crafting, stockpiles, resource gathering
 - [ ] Needs, mood, stress systems
 - [ ] Job assignment and task automation
-- [ ] Building and construction (beyond debug rabbit digging)
+- [ ] Building and construction (generic digging in place; no task system yet)
 - [ ] Combat and squad management
 - [ ] Item persistence and inventory
 - [ ] Dynamic chunk loading (player/NPC-centered, not just radius-at-startup)