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/sfxbank.js

@@ -115,7 +115,7 @@ const SfxBank = {
     if (this.buffers.has(name) || this.pending.has(name) || this.failed.has(name)) return;
     const file = this.FILES[name];
     if (!file || !this.ctx || typeof fetch !== 'function') return;
-    const job = fetch('/assets/sfx/' + encodeURIComponent(file) + '?v=37')
+    const job = fetch('/assets/sfx/' + encodeURIComponent(file) + '?v=38')
       .then(r => { if (!r.ok) throw new Error('HTTP ' + r.status); return r.arrayBuffer(); })
       .then(bytes => this.ctx.decodeAudioData(bytes))
       .then(buffer => this.buffers.set(name, this.trim(buffer)))