fix(assembly): use instance suffixes for duplicate part labels

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

tests/src/Base/UniqueNameManager.cpp

@@ -122,4 +122,15 @@ TEST(UniqueNameManager, UniqueNameWith9NDigits)
     manager.addExactName("Compound123456789");
     EXPECT_EQ(manager.makeUniqueName("Compound", 3), "Compound123456790");
 }
+TEST(UniqueNameManager, StructuredPartNumberDecomposition)
+{
+    // Structured part numbers like P03-0001 have their trailing digits
+    // treated as the uniquifying suffix by UniqueNameManager. This is
+    // correct for default FreeCAD objects (Body -> Body001) but wrong
+    // for structured identifiers. Assembly module handles this separately
+    // via makeInstanceLabel which appends -N instance suffixes instead.
+    Base::UniqueNameManager manager;
+    manager.addExactName("P03-0001");
+    EXPECT_EQ(manager.makeUniqueName("P03-0001", 3), "P03-0002");
+}
 // NOLINTEND(cppcoreguidelines-*,readability-*)