feat: Group NPC Generator tool

Four NPC types: Carrion, Serial Killer, Monstaret, Machine
Each with SLA-appropriate stat ranges, name generators, ability pools.

Features:
- Type selector tabs (colour-coded per type)
- Count 1-10, Generate / Add More / Clear All
- Compact 2-column grid of NPC cards
- Inline editable: name, STR/AGI/WIT/EMP, HP (current/max), ARM, DMG, notes
- +/- HP buttons with live colour bar (green > yellow > red)
- Defeat toggle (grey out) and Reset HP to max
- Reroll individual NPC (preserves notes)
- Create as World Actors button
- Green "Group NPCs" button in Actor Directory (GM only)

v1.5.2

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": "1.5.1",
+  "version": "1.5.2",
   "compatibility": {
     "minimum": 13,
     "verified": "14.359"