feat(settings): SYSVAR_CATALOG + system_variable_describe; system_set_variable refuses out-of-range values

90 authored rows (type, range/enum, default, meaning, saved-in, per-engine
support, friendly key); every ezdxf claim is checked against the real
backend and the registry-saved set is pinned to what the backend refuses.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

docs/tool-inventory.json

@@ -3,7 +3,7 @@
   "schema_version": 1,
   "package_version": "1.5.1",
   "totals": {
-    "tools": 166,
+    "tools": 167,
     "resources": 8,
     "prompts": 5,
     "groups": 20
@@ -24,7 +24,7 @@
     "pid": 9,
     "premium": 12,
     "solids": 5,
-    "system": 7,
+    "system": 8,
     "templates": 2,
     "transactions": 3,
     "validation": 1,
@@ -2099,6 +2099,20 @@
         "readOnlyHint": true
       }
     },
+    {
+      "name": "system_variable_describe",
+      "group": "system",
+      "cost": "read",
+      "summary": "Explain a system variable: type, range, default, where it is saved, which engine honours it.",
+      "tags": [
+        "settings",
+        "system"
+      ],
+      "annotations": {
+        "title": "Describe System Variable",
+        "readOnlyHint": true
+      }
+    },
     {
       "name": "template_apply_layers",
       "group": "templates",