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

tests/test_tool_registry.py

@@ -517,8 +517,10 @@ async def test_tool_group_sizes_are_unchanged():
     when `pid_tag_parse` (the ISA-5.1 tag grammar as a read-only tool)
     joined; the `pid`
     tag is ranked first in `_GROUP_TAG_PRIORITY` so those tools file under
-    `pid` rather than under their secondary `query` / `create` tags. Every
-    other number here has been unchanged since the snapshot was taken.
+    `pid` rather than under their secondary `query` / `create` tags. `system`
+    moved 7 -> 8 when v1.6's `system_variable_describe` (the sysvar catalogue
+    as a read-only tool) opened SECTION 20. Every other number here has been
+    unchanged since the snapshot was taken.
     """
     sizes = {label: len(names) for label, names in (await server._tool_groups()).items()}
     assert sizes == {
@@ -537,10 +539,10 @@ async def test_tool_group_sizes_are_unchanged():
         "pid": 9,
         "premium": 12,
         "solids": 5,
-        "system": 7,
+        "system": 8,
         "templates": 2,
         "transactions": 3,
         "validation": 1,
         "view": 4,
     }
-    assert sum(sizes.values()) == 166
+    assert sum(sizes.values()) == 167