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

discovery/aliases.py

@@ -2465,6 +2465,19 @@ TOOL_ALIASES: dict[str, ToolAliases] = {
             "mimari plan",
         ),
     ),
+    # ── Understanding & QA (SECTION 26, track H group D) ────────────────────
+    "drawing_diff": ToolAliases(
+        acad=("COMPARE",),
+        synonyms=(
+            "compare two drawings",
+            "compare two revisions",
+            "what changed between revisions",
+            "drawing differences",
+            "cloud the changes",
+            "revizyonlar arası değişiklik",
+            "revizyonlar arasi degisiklik",
+        ),
+    ),
 }