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.

flake.lock

@@ -8,11 +8,11 @@
         "rust-analyzer-src": "rust-analyzer-src"
       },
       "locked": {
-        "lastModified": 1775462255,
-        "narHash": "sha256-YRzdvh6nvMebcgO2nDpr8dqVwKHpp1BBRUHeMxX9UAY=",
+        "lastModified": 1779612045,
+        "narHash": "sha256-+7lfNVnmXJDkiRYHd5NoNwYoyUcc0LcXPaIJqjO7VWM=",
         "owner": "nix-community",
         "repo": "fenix",
-        "rev": "f90343f1ed330243d4bbdbce51acbd93b776a797",
+        "rev": "d7be747f0a65af378de515fc3cee131bf99a008f",
         "type": "github"
       },
       "original": {
@@ -31,11 +31,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1769799857,
-        "narHash": "sha256-88IFXZ7Sa1vxbz5pty0Io5qEaMQMMUPMonLa3Ls/ss4=",
+        "lastModified": 1778151388,
+        "narHash": "sha256-lldMJPUeouEjO8/7aLuwhcsIw29vVihm2ZALzjiqfec=",
         "owner": "nix-community",
         "repo": "naersk",
-        "rev": "9d4ed44d8b8cecdceb1d6fd76e74123d90ae6339",
+        "rev": "efdddff9ff4d8e7d0056d57ec67dac50f75ab8f6",
         "type": "github"
       },
       "original": {
@@ -46,11 +46,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1775036866,
-        "narHash": "sha256-ZojAnPuCdy657PbTq5V0Y+AHKhZAIwSIT2cb8UgAz/U=",
+        "lastModified": 1779508470,
+        "narHash": "sha256-Ap9KJX+5xHIn3bPIpfNgT6MEXdAECECwo4/rmlQD74M=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "6201e203d09599479a3b3450ed24fa81537ebc4e",
+        "rev": "29916453413845e54a65b8a1cf996842300cd299",
         "type": "github"
       },
       "original": {
@@ -70,11 +70,11 @@
     "rust-analyzer-src": {
       "flake": false,
       "locked": {
-        "lastModified": 1775429583,
-        "narHash": "sha256-bFC/p7Ywyd9QIr9DbU3Q75c7AcaCm9wVmEvcI3702cY=",
+        "lastModified": 1779569060,
+        "narHash": "sha256-NSnk5D+3KEfRdbgPijs33N2RAKSG6A74SwfnynLcouo=",
         "owner": "rust-lang",
         "repo": "rust-analyzer",
-        "rev": "38fb8f92ac15853d7fa9fb47fc2d81fdd5cd6c7e",
+        "rev": "987ea33645ab1c709b1df6823038abcb2fe8973e",
         "type": "github"
       },
       "original": {