fix: weapon roll crash on NPC actors with embedded weapons

NPC actors have flat attribute numbers (e.g. strength: 3) not objects
({value: 3}) and no system.skills object at all. Both _onWeaponRoll
and _onRoll were crashing when Group NPC-generated actors used their
attack buttons.

Fix:
- Read attributes via typeof check — flat number used directly,
  object uses .value — so NPC stat values are used correctly
- skills accessed via system.skills?.[skill] optional chaining so
  missing skills object returns 0 rather than throwing TypeError

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": "1.5.4",
+  "version": "1.5.5",
   "compatibility": {
     "minimum": 13,
     "verified": "14.359"