Rename iron to metal, add mining and recruitment

- Updated CLI, README and documentation to use `--starting-metal` with
  `--starting-iron` retained as an alias.
- Introduced `Resources` struct and constants for wood delivery and mine
  production.
- Added new structures `Mine` and `MineConstruction` with cost,
  production
  and cancel logic.
- Implemented building view for constructing mines and recruitment view
  for spawning miners.
- Extended keybindings with `Building` and `Recruitment` actions and
  updated
  action availability logic.
- Modified game state, board, units and UI widgets to handle metal
  resources
  and new construction mechanics.
- Updated tests and increased total test count to 82, covering
  resources,
  construction, recruitment and context‑aware keybindings.

tests/cli.rs

@@ -19,7 +19,7 @@ fn numeric_options_accept_boundaries_and_reject_out_of_range_values() {
     ("--map-width", 36, 108),
     ("--map-height", 11, 39),
     ("--starting-wood", 1, 150),
-    ("--starting-iron", 1, 150),
+    ("--starting-metal", 1, 150),
     ("--supply-limit", 49, 149),
     ("--skill-points-limit", 90, 690),
   ] {