Item system overhaul: rarity bands, per-item caps, source pools + 14 new passives

- Add ITEM_RARITY (common/uncommon/rare/legendary) with per-item tier caps
- Weighted pools per source: elites favour common, item rooms uncommon,
  bosses rare/legendary; legendaries are boss-exclusive; capped items excluded
- Rarity-weighted powerScore so deep common stacks inflate difficulty less
- Differentiate duplicates: Grafted Trigger, Crow Bait, Hollow Bones, Twin Hearts
- 14 new clean passives incl. Chill Gland (activates slowOnHit) and Hooked Sinew
  (activates magnetPull); revive magnetPull in pickups
- Rarity-tinted pedestal glow + nameplates, rarity-sorted HUD implant tray,
  dynamic help implant pages (8 -> 9)
- Generate 14 item icons via gen_assets.py; bump asset cache to v=38
- Update smoke tests for 60 items, per-item caps, pools, weighted power

js/bosses.js

@@ -593,7 +593,7 @@ function onBossDeath(e) {
   addScore(500 + G.floor * 100);
   addShake(12);
   // boss reward: item pedestal + a real weapon drop + stairs down
-  spawnItemPedestal(W / 2 - 90, H / 2);
+  spawnItemPedestal(W / 2 - 90, H / 2, null, 'boss');
   const bw = rollWeaponDrop(G.floor + 2); // bosses drop the good stuff
   spawnPickup('weapon', W / 2 + 90, H / 2, { wid: bw.id });
   spawnPickup('stairs', W / 2, H / 2 + 80);