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

tests/test_tool_registry.py

@@ -495,7 +495,7 @@ async def test_tool_groups_is_byte_identical_to_the_source_declared_grouping():
 
 
 async def test_tool_group_sizes_are_unchanged():
-    """Frozen snapshot of the surface (241 tools, 26 groups).
+    """Frozen snapshot of the surface (242 tools, 26 groups).
 
     Taken before @cad_tool landed at 131 tools; `batch` moved 2 -> 3 when
     v1.5.0's `cad_batch` joined `entity_batch_create`/`entity_batch_modify`, and
@@ -591,10 +591,16 @@ async def test_tool_group_sizes_are_unchanged():
     exactly `PACK_TOOL_NAMES["arch"]`, and every group the track-F branches
     had grown while the tag was unranked on them returned to its
     pre-track-F value.
+
+    Track H group D opened its copy of SECTION 26 (Understanding & QA) with
+    `drawing_diff`, tagged `analysis` and `query`; `analysis` outranks `query`
+    in `_GROUP_TAG_PRIORITY`, so it files under `analysis` (12 -> 13). This
+    snapshot is branch-local: the track H merge recomputes it once the
+    understand, network and diff branches have landed.
     """
     sizes = {label: len(names) for label, names in (await server._tool_groups()).items()}
     assert sizes == {
-        "analysis": 12,
+        "analysis": 13,
         "architecture": 12,
         "batch": 3,
         "blocks": 9,
@@ -621,4 +627,4 @@ async def test_tool_group_sizes_are_unchanged():
         "validation": 1,
         "view": 4,
     }
-    assert sum(sizes.values()) == 241
+    assert sum(sizes.values()) == 242