feat: SLA-specific art for generated NPCs, weapons, armor, abilities

Copied 37 SLA art assets from Data/assets/SLA_Assets into
systems/zero-engine/assets/npc-art/ so they deploy with the system
to remote servers via GitHub pull.

NPC portraits (from Species folder):
  carrion.png        — Advanced Carrion
  streetgang.png     — Human
  serialkiller.png   — Brain Waster
  monstaret.png      — Stormer 313
  machine.png        — Stormer 711

Weapons (SLA weapon art):
  pistol, smg, shotgun, rifle, sniper, knife, combat-knife,
  blade, sabre, baton, club, chainsaw, hatchet,
  claws, natural-weapon, heavy

Armor (by ARM rating):
  street.png (0-1), light.png (1-2), heavy.png (3-4), exo.png (5+)

Abilities (trait/special art):
  attack, psychosis, hard-hitting, fast-reflexes, menacing,
  regeneration, enemy, contagion

All icon helper functions updated to use SLA art paths.

system.json

@@ -2,7 +2,7 @@
   "id": "zero-engine",
   "title": "Zero Engine (SLA Industries)",
   "description": "A basic game system for SLA Industries using the Zero Engine rules.",
-  "version": "1.5.6",
+  "version": "1.5.7",
   "compatibility": {
     "minimum": 13,
     "verified": "14.359"