Add signature gemstone as guaranteed local buy in every shop

- Ensure each class's gemstone (tier 3–4 producer good) always appears on the shop floor, swapping into the least-important slot when at max stock to preserve size and avoid duplicates.
- Add a test sweeping every system/class to verify the world's "jewel" is present, produced, and premium in all shops.

dev/economy.test.mjs

@@ -189,6 +189,25 @@ const classFor = (sysId, good, list) => (list ?? []).find((c) => g.economy.class
   const cshop = E.shopFor('capital', capSys, g);
   check('a capital buys back EVERYTHING (universal sink)', cshop.sell.length === goods.length);
   check('a capital floor is 8–16 goods', cshop.buy.length >= 8 && cshop.buy.length <= 16);
+
+  // THE WORLD'S JEWEL (§4/§6): every shop carries ITS class's signature gem —
+  // a guaranteed premium good + a guaranteed local buy in every shop. Sweep
+  // every system of every class (a gem that's only present by catalog luck
+  // would fail here: the floor must always include the class's gemstone).
+  let gemWorlds = 0;
+  let gemMissing = 0;
+  for (const [id, set] of g.economy.classBySystem) {
+    for (const cls of set) {
+      const gem = goods.find((x) => x.category === 'gemstone' && Array.isArray(x.producer) && x.producer.includes(cls));
+      if (!gem) continue;
+      gemWorlds++;
+      const shop = E.shopFor(cls, id, g);
+      const row = shop.buy.find((b) => b.id === gem.id);
+      // the jewel is a premium good (tier 3–4) + a local buy (produced).
+      if (!row || row.produced !== true || (Number(gem.tier) || 0) < 3) gemMissing++;
+    }
+  }
+  check(`every shop carries its class's signature gem (${gemWorlds - gemMissing}/${gemWorlds} worlds)`, gemWorlds > 0 && gemMissing === 0);
 }
 
 // ---------------------------------------------------------------------------

js/economy/Economy.js

@@ -184,6 +184,8 @@ function seededSelect(items, n, seed, cls, systemId) {
  *          · plain world: its produced goods (abundant, cheap)
  *          · station: produced (≤ half) + a seeded selection of resold goods (variety)
  *          · capital: produced (≤ half) + a seeded selection of premium goods (tier 3–4)
+ *          · the class's SIGNATURE GEM is ALWAYS on the floor (the world's
+ *            "jewel" — a guaranteed premium good + a guaranteed local buy)
  *   sell — goods the shop BUYS BACK (what you can sell), each { id, price }
  *          · plain world / station: the goods the class wants
  *          · capital: EVERYTHING (the universal sink)
@@ -227,6 +229,20 @@ export function shopFor(cls, systemId, galaxy) {
     }
   }
 
+  // THE WORLD'S JEWEL (goods.json gemstones, §4/§6): the class's signature
+  // gem is ALWAYS on the shelf — a guaranteed premium good (tier 3–4) and a
+  // guaranteed local buy in every shop, whatever the seeded floor picked.
+  // It's a produced good of `cls`, so it prices at the producer discount;
+  // swap it into the least-important slot (a hub's resold filler, or an
+  // extra local good on a plain world) keeping the 8–16 floor size + no dups.
+  const gem = goods.find(
+    (g) => g.category === 'gemstone' && Array.isArray(g.producer) && g.producer.includes(cls),
+  );
+  if (gem && !buyIds.includes(gem.id)) {
+    if (buyIds.length < maxStock) buyIds.push(gem.id);
+    else buyIds[buyIds.length - 1] = gem.id;
+  }
+
   const buy = buyIds.map((id) => ({
     id,
     name: goodById(id)?.name ?? id,