kernel-cases: 2928's base extrudes the ring it describes

The case's base is a stadium ring (r 3..5 arcs, centres 20 apart, z 0..2),
as its description and expected volume (2 (80 + 16 pi) for the base) say,
but its extrude picked no profile, and "all profiles" is every drawn region:
the ring and the stadium island inside it. The base came out filled and the
join 1066 mm3. SMLib's union was exact for those operands. The extrude now
picks the ring (the region at (4, 0)).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/kernel-cases/booleans/2928-join-cap-touching-lips-with-offset-walls/recipe.json

@@ -131,6 +131,14 @@
    "command": "solid.extrude",
    "params": {
     "sketch": "Base",
+    "profiles": [
+     {
+      "point": [
+       4,
+       0
+      ]
+     }
+    ],
     "distance": 2,
     "operation": "new",
     "body_name": "Base"