Refactor skirmish tasks and update dependencies

- Disable automatic board.advance_turn and replace it with explicit unit
  movement handling.
- Add Tunnel structure creation during digging tasks.
- Extend DiggingTask with pop_front and current_target helpers.
- Rename task getters to get_ref_tasks and add mutable get_mut_tasks.
- Update Unit trait and Units enum to expose these new methods.
- Remove unused EMPTY_TASKS lazy static.
- Bump flake.lock revisions, hashes, and timestamps for several
  dependencies.

src/app/states/skirmish.rs

@@ -11,7 +11,7 @@ pub struct SkirmishState {
 
 impl SkirmishState {
     pub fn tick_update(&mut self) {
-        self.board.advance_turn();
+        // self.board.advance_turn();
 
         // if self.board.is_victory() {}