Knight fortresses: knight towers, patrols, alarm, claimable knight ground and vaults

- fortress.rs: fortress builder (masonry walls with doors, courtyard, corner towers, vault chambers
  full of mixed treasure, shard chamber, gate of the order as the heart, guards, patrols)
- Knight towers shoot the player's units; creatures tear down knight towers within reach
- Patrols walk their round; guards and patrols that spot an enemy alarm the others nearby
- Imps claim knight ground (slower, not next to fighting heroes); vaults hand over their treasure,
  shard chambers give the shard back; claiming alarms the knights
- Knights tear down the player's bridges near their posts
- Fix: units next to a tower could stop at the tile edge just out of melee reach
- Tests for towers, vault looting, patrols/alarm and bridge tearing

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main.rs

@@ -2,6 +2,7 @@
 
 mod audio;
 mod cloud;
+mod fortress;
 mod game;
 mod gfx;
 mod i18n;