Building funds safe from raids; settlement tiers; population and food on the town square

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

README.md

@@ -16,8 +16,12 @@ Alpha. One season, bandit raids plus two friendly events, six buildings.
 - **Bank:** 12 slots of up to 1,000 each to start. When something doesn't
   fit it goes to the city stockpile (or the action stops, if you prefer).
   Raids never touch your bank. Players can give items to each other.
-- **City:** donations fill the stockpile and treasury; anyone can start a
-  building once they cover its cost. Population eats food and pays taxes.
+- **City:** each building collects its own fund, from players' banks or the
+  city's stockpile and treasury. Funds are safe from raiders, and a funded
+  building joins the construction queue by itself. Population eats food,
+  pays taxes, and grows up to the housing cap; it promotes the settlement
+  (hamlet, village at 40, town at 90, city at 150), which raises building
+  level caps and gives everyone bank slots.
 - **Events:** raids are announced a day or two ahead. Intel (spymaster's
   guild + scout reconnaissance) narrows down when, how strong, and finally
   where their camp is, so fighters can sabotage it. Defeat kills or drives