fix(com): hatch islands on a live seat — typed edges become temporary entities

hatch_add_boundary was refused on COM (hatch_edge_paths), so a transverse
section of any hollow part — a bushing, a flange bore — could not be drawn in
AutoCAD at all: draw_prims asked the capability map and refused up front.

ActiveX takes a loop as whole objects, which is not the same as "cannot take a
typed edge": each edge now becomes a real temporary LINE, true ARC or ELLIPSE
in the hatch's own space, appended with AppendInnerLoop, evaluated and
deleted. A loop that is only chained lines becomes one closed polyline — one
COM call instead of 180 for a flattened bore. The edge list is validated in
full first with the headless engine's messages; a loop AutoCAD refuses is
taken back out with its temporaries.

Measured on AutoCAD 2026 (25.1s), in scratch documents closed unsaved:
- a two-arc island leaves exactly 10000 - 100*pi (9685.8407) of a 100 x 100 hatch;
- the OD40/ID20 sleeve's transverse section B-B, drawn through the view engine
  and draw_prims, hatches 942.2864 mm2 — the engine's declared 180-gon;
- an open chain is refused (E_FAIL) and the entity count is unchanged.

hatch_edge_paths is now supported on both engines.

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

engineering/mech/draw.py

@@ -216,10 +216,12 @@ async def _ensure_layers(backend, names) -> tuple[str, ...]:
 def _refuse_islands_without_edge_paths(backend, prims) -> None:
     """Refuse a hatch with islands on an engine that cannot attach them - before drawing.
 
-    Islands go through `hatch_add_boundary`, which the live engine refuses
-    (capability ``hatch_edge_paths``: ActiveX appends loops as objects, not
-    typed edges). Finding that out at the hatch would leave the rest of the
-    view already drawn; asking the capability map first leaves nothing behind.
+    Islands go through `hatch_add_boundary` (capability ``hatch_edge_paths``).
+    Both shipped engines support it - the live one turns each typed edge into a
+    temporary LINE/ARC/ELLIPSE and appends it with ``AppendInnerLoop`` - but a
+    backend that declares it unsupported must say so before anything is drawn:
+    finding out at the hatch would leave the rest of the view already on the
+    sheet, while asking the capability map first leaves nothing behind.
     """
     if not any(isinstance(p, HatchArea) and len(p.loops) > 1 for p in prims):
         return