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

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=40')
+    const job = fetch('/assets/sfx/' + encodeURIComponent(file) + '?v=41')
       .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)))