fix(understand): drawing_diff minors from the review — direction, repeated tags, bounded output

The review of Task 9 left four minor findings. Three change what a user reads:

- A LINE drawn back to front and moved was reported as a geometry change.
  The signature already ignores a LINE's direction, so its movement now
  does too: the reversed order is tried before the entity is called
  reshaped.
- An INSERT carrying one attribute tag twice was read as a dict. An edit
  of the first value could disappear, and since the pair then had nothing
  to report, it counted as unchanged. A repeated tag is now compared as
  the list of its values; a tag carried once keeps plain from / to.
- cap_report capped the three lists but not the clusters. A widespread
  edit of a large drawing could return an unbounded cluster list. It is
  capped like the others, with truncated["clusters"]; the markup still
  clouds every cluster.

The fourth bounds work: when the whole drawing moves, every entity reaches
the position stage, and a generous move_tol gave each one hundreds of
candidates, which is n x m memory. Only the 32 nearest per entity are kept
now. A 200-line test shows each line still pairs with its own moved copy.

The first three tests failed against the previous module (the attribute
test with no changed row at all). The whole diff suite passes: 38 tests.

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

tests/test_understand_diff_tool.py

@@ -46,7 +46,7 @@ async def test_the_current_document_against_its_older_revision(client, tmp_path)
         pair["insert"],
     }
     assert result["blocks"]["compared"] is True and result["blocks"]["changed"] == []
-    assert result["truncated"] == {"changed": 0, "added": 0, "removed": 0}
+    assert result["truncated"] == {"changed": 0, "added": 0, "removed": 0, "clusters": 0}
     assert await _total(client) == before, "a diff without markup writes nothing"