When parts with structured part numbers (e.g., P03-0001) are inserted
into an assembly multiple times, UniqueNameManager::decomposeName()
treats the trailing digits as an auto-generated suffix and increments
them (P03-0002, P03-0003), corrupting the part number.
Add a makeInstanceLabel() helper in AssemblyLink.cpp that appends -N
instance suffixes instead (P03-0001-1, P03-0001-2). All instances get
a suffix starting at -1 so the original part number is never modified.
Applied at all three Label.setValue() sites in
synchronizeComponents() (AssemblyLink, link group, and regular link
creation paths).
Also add a UniqueNameManager test documenting the trailing-digit
decomposition behavior for structured part numbers.
Closes#327
