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

discovery/aliases.py

@@ -1313,6 +1313,18 @@ TOOL_ALIASES: dict[str, ToolAliases] = {
             "health check",
         ),
     ),
+    "system_variable_describe": ToolAliases(
+        acad=("SETVAR", "SYSVARMONITOR", "SYSVDLG"),
+        synonyms=(
+            "sysvar",
+            "what does ltscale do",
+            "explain a system variable",
+            "system variable reference",
+            "which variables are saved in the drawing",
+            "valid range of a system variable",
+            "list system variables",
+        ),
+    ),
     # ── Transactions ────────────────────────────────────────────────────────
     "transaction_begin": ToolAliases(
         synonyms=(