This patch renames nine structure icons for consistency and updates the templates that use them.
* `siege_workshop.png` → `arsenal.png`
* `blacksmith.png` → `forge.png`
* `palace.png` → `pers_palace.png` (because it's an explicitly Persian symbol)
* `pers_stable.png` → `stable.png` (because it's also used for non-Persian stables)
* `defense_tower.png` → `tower_large.png`
* `sentry_tower.png` → `tower_small.png`
* `gate.png` → `wall_gate.png`
* `tower.png` → `wall_tower.png`