In hierarchical KiCad designs, sub-sheets (.kicad_sch files referenced
via (sheet ...) blocks) do not have a (sheet_instances) section — that
only appears in the top-level schematic.
WireManager.add_label was returning False with a logged error whenever
no (sheet_instances) marker was found, making it impossible to add any
net label (local, global, or hierarchical) to a sub-sheet.
Fix: fall back to inserting before the final item of the s-expression
list, which is equivalent to appending before the closing ')' of the
outer (kicad_sch ...) block.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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