feat(understand): drawing_diff - two revisions matched by geometric signature

engineering/understand/diff.py reads each revision once into entity
records and matches them in three stages: exact signature (type, layer,
geometry rounded to tol, text, block, rotation, scale, attributes;
handles ignored), then handle while at least half of the exact matches
kept theirs, then type + layer + nearest anchor within move_tol (default
2 % of the 1st-99th percentile diagonal). The report counts changes by
layer and type, says what changed per entity (moved by, text, attribute,
radius ... from -> to), compares named block definitions by content and
groups the changes into clusters.

drawing_diff opens group D's copy of SECTION 26. It is read-only unless
markup=True, which files revision clouds around the model-space clusters
on the current document through add_revision - refused before any read
with new_path, without a valid rev, or on an engine without revcloud.
A revision path that is neither .dxf nor .dwg is refused naming the
parameter (old_path: / new_path:), like every other path refusal.

Tests: the synthetic plant P&ID edited five ways (a move, a text, an
attribute, an add, a delete) under renumbered handles reports exactly
those five edits; renumbering alone reports nothing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

docs/tool-inventory.json

@@ -3,13 +3,13 @@
   "schema_version": 1,
   "package_version": "1.5.1",
   "totals": {
-    "tools": 241,
+    "tools": 242,
     "resources": 8,
     "prompts": 5,
     "groups": 26
   },
   "groups": {
-    "analysis": 12,
+    "analysis": 13,
     "architecture": 12,
     "batch": 3,
     "blocks": 9,
@@ -897,6 +897,20 @@
         "destructiveHint": true
       }
     },
+    {
+      "name": "drawing_diff",
+      "group": "analysis",
+      "cost": "mutate",
+      "summary": "Compare two revisions of a drawing: what moved, changed, was added or removed.",
+      "tags": [
+        "analysis",
+        "query"
+      ],
+      "annotations": {
+        "title": "Drawing: Diff Two Revisions",
+        "destructiveHint": false
+      }
+    },
     {
       "name": "drawing_export_dwg",
       "group": "sheet",