Gave the world a simple economy: one pool per nation and a global market

The simple economy is now the default and the old simulation is frozen as the hard model, selectable per game. A nation keeps one resource pool available in every region, fed by public production works rather than private per-region stores, and construction and training begin at once, paced by the region's spare power instead of gathering materials over days.

Money is euro only: a per-inhabitant daily tax funds a treasury spent in a global market, where buy and sell lots trade against the world stock and move its price. The hard economy keeps its per-nation currencies, central banks and exchange rates.

Simple games skip the price-settling warmup and economic migration; the snapshot carries the economy model, the market and the pool figures the client draws. Added the matching test suites and the design briefs under docs/simple_economy.

DESIGN.md

@@ -8,6 +8,7 @@ context on purpose.
 | --- | --- |
 | `ICONS.md` | Replacing the placeholder "dummy icon" SVGs with real art. |
 | `SIMPLIFICATIONS.md` | The `[~]` roadmap items: deliberate simplifications worth revisiting. |
+| `docs/simple_economy/` | The simple economy replacement, one implementation prompt per module. |
 
 `ROADMAP.md` remains the authoritative task list; these docs add the *why* and
 the design space around the open items.