fix: three measurement defects the adversarial triage surfaced

**COM discarded areas it had already read.** `entity_measure` fell back to
rebuilding geometry from `.Coordinates` when `.Area` *or* `.Length` was
missing, coupling two independent property reads. HATCH, REGION and 3DSOLID
expose `.Area` and neither of the others, so the fallback fired, found no
vertices, and raised "AutoCAD reported neither an area nor usable vertices" --
while holding the area. Those are the exact types the README sells the COM
backend on. Now only a missing `.Area` triggers the fallback; an absent
perimeter is reported as `None` with `perimeter_exact: False` instead of being
invented, which also removes a `round(None, 6)` TypeError waiting for the first
type that answers `.Area` and nothing else.

**A plain square hatch reported itself self-intersecting.** `ezdxf.path
.flattening()` repeats the start point, and `is_self_intersecting` excluded
adjacent edges *by index*, so with the duplicate present the first and last
real edges were cross-tested and registered as crossing. Measured: a 40x40
SOLID hatch came back `area: 1600.0, self_intersecting: True`. A closed loop
now answers the same whether or not it carries its closing vertex, and a real
bowtie is still caught (`area: 0.0, self_intersecting: True`).

**The bowtie warning never fired on the live backend.** `self_intersecting` was
only computed inside the COM fallback, so an LWPOLYLINE bowtie -- which has
both `.Area` and `.Length` and never reaches it -- returned `area: 0.0,
self_intersecting: null`. The confident zero this field exists to flag, going
out unflagged. It is now computed on the normal path too.

Also corrects a docstring written earlier today in `contracts/analysis.py`
claiming the live backend "asks AutoCAD for `.Area`, which has already applied
the [hatch island] style". Whether ActiveX's hatch area applies the style is
not verified against a live seat, and the method could not measure a hatch at
all. It now says so.

The ActiveX member profiles behind the COM tests are assumptions, not facts,
and the tests say so; a live seat settles them. `.Circumference` for CIRCLE and
`.Perimeter` for REGION are deliberately NOT added here -- writing more members
from the reference is what produced four wrong beliefs in v1.4/v1.5.

1267 passed, 1 xfailed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_measure.py

@@ -198,3 +198,39 @@ def test_two_straight_vertices_still_enclose_nothing():
 
 def test_a_single_vertex_is_still_degenerate():
     assert polygon_area_perimeter([(0.0, 0.0, 1.0)], closed=True) == (0.0, 0.0)
+
+
+# ── a closed loop must answer the same however it was handed over ───────────
+
+
+def test_a_repeated_closing_vertex_is_not_a_self_intersection():
+    """The false alarm this shipped with.
+
+    `ezdxf.path.flattening()` repeats the start point, so every hatch boundary
+    and every flattened curve arrived here with a duplicate closing vertex. The
+    adjacency guard excludes neighbours *by index*, so the first and last real
+    edges — which share a point but not an index neighbourhood — were
+    cross-tested and reported as crossing. A plain 40x40 square hatch came back
+    `self_intersecting: True`: a confident wrong warning on the field that
+    exists to warn.
+    """
+    square = [(0.0, 0.0), (50.0, 0.0), (50.0, 50.0), (0.0, 50.0)]
+
+    assert is_self_intersecting(square) is False
+    assert is_self_intersecting([*square, (0.0, 0.0)]) is False
+
+
+def test_a_bowtie_is_still_caught_when_it_carries_a_closing_vertex():
+    """Dropping the duplicate must not disarm the check."""
+    bowtie = [(0.0, 0.0), (50.0, 50.0), (50.0, 0.0), (0.0, 50.0)]
+
+    assert is_self_intersecting(bowtie) is True
+    assert is_self_intersecting([*bowtie, (0.0, 0.0)]) is True
+
+
+def test_a_triangle_with_a_closing_vertex_is_still_not_degenerate():
+    """Four entries, three real corners — must not fall under the `< 4` guard
+    and answer False for the wrong reason."""
+    triangle = [(0.0, 0.0), (60.0, 0.0), (30.0, 40.0), (0.0, 0.0)]
+
+    assert is_self_intersecting(triangle) is False