Fix face Extrude Join targets in assemblies (#1375)

A new face-based Extrude Join (and outward Press Pull) joins the picked face's body by default and stores that scope, so an edge-touching neighbour is no longer tried first or absorbed. Explicit targets and participants still win. Related to #226.

tests/action_matrix/designs/multi-body-face-extrude.json

@@ -0,0 +1,7 @@
+{
+  "description": "Issue #226: the upper cap precedes the wall in body order and touches only the edge of a 10 mm outward extrusion from the wall's x=40 face.",
+  "commands": [
+    {"command":"solid.box","params":{"corner":[0,0,20],"length":40,"width":30,"height":5,"body_name":"Cap"}},
+    {"command":"solid.box","params":{"length":40,"width":30,"height":20,"body_name":"Wall"}}
+  ]
+}