fix(understand): a truncated DXF is refused by the snapshot, not left hanging

ezdxf's tag reader raises StopIteration on a file that ends early, for example
after an interrupted save or a partial copy. read_snapshot caught only OSError
and DXFError, so the StopIteration escaped. asyncio.to_thread cannot hand
StopIteration back to an awaiting caller, so every track H tool reading such
a file would have hung instead of refusing it.

The review of drawing_diff found this in its own reader. The snapshot now
does the same: every parse or walk failure except MemoryError becomes the
existing "is not a readable DXF file" refusal. A new test checks this both
directly and through a worker thread with a 10 s timeout. It failed with
StopIteration before this change and passes after it.

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

tests/test_understand_snapshot.py

@@ -8,6 +8,7 @@ behaves the way AutoCAD 2026 was measured to behave (the extension must be
 
 from __future__ import annotations
 
+import asyncio
 import math
 from pathlib import Path
 
@@ -173,6 +174,23 @@ def test_read_snapshot_refuses_missing_and_unreadable_files(tmp_path):
     assert "not a readable DXF" in str(excinfo.value)
 
 
+def test_a_truncated_dxf_is_refused_not_left_hanging(tmp_path):
+    # An interrupted save: the header opens and the file ends. ezdxf's tag reader
+    # raises StopIteration here, which asyncio.to_thread cannot hand back to an
+    # awaiting tool - the call would never answer. It must be the same refusal
+    # as any other unreadable file, also through a worker thread.
+    half = tmp_path / "half.dxf"
+    half.write_text("  0\nSECTION\n  2\nHEADER\n  9\n$ACADVER\n  1\nAC1015\n", encoding="utf-8")
+    with pytest.raises(ValueError, match="not a readable DXF"):
+        read_snapshot(str(half))
+
+    async def through_a_thread():
+        return await asyncio.wait_for(asyncio.to_thread(read_snapshot, str(half)), timeout=10)
+
+    with pytest.raises(ValueError, match="not a readable DXF"):
+        asyncio.run(through_a_thread())
+
+
 # -- take_snapshot on the headless engine ---------------------------------------