Add support structures and stress mechanics

Introduce support structures (roof supports) and a stress system that
propagates from bases through tunnels, causing collapses when stress
reaches 100 %.  Add auto‑support feature with toggleable tasks, new
keybindings (Tasks, AutoSupports, Stress), and a support‑building tab.
Implement collapsed tunnels, support work progress, and related UI
rendering (stress view, auto‑support indicators).  Extend board resize
logic, update game state structs, and adjust keybinding actions.  Update
documentation (README, AGENTS, CODE_MAP, PROGRESS, TESTING) to reflect
new features.  Add flake.nix dependencies (bash, gitea‑actions‑runner,
docker).  Add extensive tests for tunnel stress, support building,
auto‑supports, and related mechanics.

tests/digging.rs

@@ -362,6 +362,9 @@ fn confirmed_route_splits_into_moving_and_digging_segments() {
     for col in [3, 4, 7] {
         tunnel(&mut game.board, 5, col);
     }
+    let mut supported: Tunnel = Tunnel::new();
+    supported.set_support(true);
+    game.board.cells[5][7].set_structure(Structures::Tunnel(supported));
     game.board.change_focused_cell(MoveFocusedCell::Left);
     game.board.toggle_marking();
     for _ in 0..5 {