WireManager.add_wire and add_polyline_wire bailed out with
"No sheet_instances section found in schematic" when called on a
hierarchical sub-sheet — that block only exists in the root .kicad_sch.
The handler in kicad_interface.py converted the False return into a
flat "Failed to add wire" message, leaving callers with no diagnostic.
Apply the same fallback that WireManager.add_label has used since the
hierarchical-design support: when (sheet_instances ...) is absent,
append the new (wire ...) item at the end of the outer (kicad_sch ...)
form.
Adds tests/test_add_wire_sub_sheet.py with 6 regression tests covering
both add_wire and add_polyline_wire on a sub-sheet, including paren
balance, sexpdata round-trip, and segment count for an N-point polyline.
e66c13361b