Ammo economy: 25% less ammo per box, 25% fewer ammo drops

- AMMO_REFILL_SCALE 0.75 on ammoRefillFraction (smaller refills)
- AMMO_DROP_SCALE 0.75 on all three ammo spawn sites (room clear, elite,
  normal kill); help/gameplay drop-rate text updated; cache v=41

test/smoke.js

@@ -791,7 +791,7 @@ console.log('== drop economy: weapons are rare, elites feed you ==');
   }
   step(3, 16);
   check('elites always drop bonus gems (avg value ' + (eliteGemValue / 30).toFixed(1) + ')', eliteGemValue / 30 >= 5.5);
-  check('elites mostly drop ammo (' + eliteAmmo + '/30)', eliteAmmo >= 8);
+  check('elites mostly drop ammo (' + eliteAmmo + '/30)', eliteAmmo >= 6);
   check('elites sometimes drop items (' + eliteItems + '/30)', eliteItems >= 1);
   check('elites never drop weapons', eliteWeapons === 0);
   ctx.G.pickups.length = 0;