fix(task-9): no phantom changes from the tol grid, no handle stage without evidence, a truncated DXF refused

Three blocking review findings on drawing_diff.

1. An unedited entity reported as changed with changes == {} and clouded.
   The signature snaps values to the tol grid, so a value 1 ulp across a
   half-grid point lands in another bucket. AutoCAD writes 16 significant
   digits (0.14 + 0.005 = 0.14500000000000002 comes back as 0.145), so a
   round trip split the pair, which was matched later and counted as
   changed. A matched pair is now compared by difference (more than half a
   tol, around the circle for angles) instead of by grid bucket, and a pair
   with nothing to report counts as unchanged, with no row and no cluster.
   Measured live on AutoCAD 2026 (Documents.Open read-only, select all,
   Export DXF, Close(False)): before, summary changed=1, a row with
   changes {} and one cluster; after, unchanged=21, changed=0, no cluster.
   Over 12000 computed coordinates (a*0.01 + {0.005, 0.015, 0.105, 0.305})
   795 signatures flip; before, 795 were changed, and after, 0.

2. With zero exact matches, handles were declared stable, so strangers from
   two fresh files that share handles were paired and given wrong moves.
   The handle stage now needs at least one exact match. The report adds
   handle_evidence {exact_matches, kept_handle}.

3. A truncated DXF raised StopIteration from ezdxf's tag reader.
   asyncio.to_thread cannot hand that exception to an awaiting caller, so
   drawing_diff hung instead of refusing. read_revision now turns every
   parse or walk failure (not MemoryError) into ValueError "not a readable
   DXF (...)". tol is validated first, so its refusal keeps its own name.

Tests: grid splits by handle and by position, radius/height/rotation/bulge
split at tol=1, a real edit next to a split, no-evidence handles, a
truncated file directly and over the MCP wire with a 30 s timeout. The six
new tests fail against the previous module.

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

@@ -125,6 +125,20 @@ async def test_a_revision_that_is_neither_dxf_nor_dwg_is_refused_by_parameter(cl
     assert "old_path:" in text and "reads a .dxf or a .dwg" in text
 
 
+async def test_a_truncated_revision_is_refused_not_hung(client, tmp_path):
+    import asyncio
+
+    half = tmp_path / "half.dxf"
+    half.write_bytes(b"  0\nSECTION\n  2\nHEADER\n  9\n$ACADVER\n  1\nAC1015\n")
+    result = await asyncio.wait_for(
+        client.call_tool("drawing_diff", {"old_path": str(half)}, raise_on_error=False),
+        timeout=30,
+    )
+    assert result.is_error is True
+    text = result.content[0].text
+    assert "old_path:" in text and "not a readable DXF" in text
+
+
 async def test_the_diff_is_advertised_as_non_destructive(client):
     tools = {tool.name: tool for tool in await client.list_tools()}
     assert tools["drawing_diff"].annotations.destructiveHint is False