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

src/Mod/Assembly/App/AssemblyLink.cpp

@@ -311,6 +311,19 @@ void AssemblyLink::updateContents()
     purgeTouched();
 }
 
+// Generate an instance label for assembly components by appending a -N suffix.
+// All instances get a suffix (starting at -1) so that structured part numbers
+// like "P03-0001" are never mangled by UniqueNameManager's trailing-digit logic.
+static std::string makeInstanceLabel(App::Document* doc, const std::string& baseLabel)
+{
+    for (int i = 1;; ++i) {
+        std::string candidate = baseLabel + "-" + std::to_string(i);
+        if (!doc->containsLabel(candidate)) {
+            return candidate;
+        }
+    }
+}
+
 void AssemblyLink::synchronizeComponents()
 {
     App::Document* doc = getDocument();
@@ -428,7 +441,7 @@ void AssemblyLink::synchronizeComponents()
                 auto* subAsmLink = static_cast<AssemblyLink*>(newObj);
                 subAsmLink->LinkedObject.setValue(obj);
                 subAsmLink->Rigid.setValue(asmLink->Rigid.getValue());
-                subAsmLink->Label.setValue(obj->Label.getValue());
+                subAsmLink->Label.setValue(makeInstanceLabel(doc, obj->Label.getValue()));
                 addObject(subAsmLink);
                 link = subAsmLink;
             }
@@ -440,7 +453,7 @@ void AssemblyLink::synchronizeComponents()
                 );
                 newLink->LinkedObject.setValue(srcLink->getTrueLinkedObject(false));
 
-                newLink->Label.setValue(obj->Label.getValue());
+                newLink->Label.setValue(makeInstanceLabel(doc, obj->Label.getValue()));
                 addObject(newLink);
 
                 newLink->ElementCount.setValue(srcLink->ElementCount.getValue());
@@ -461,7 +474,7 @@ void AssemblyLink::synchronizeComponents()
                 App::DocumentObject* newObj = doc->addObject("App::Link", obj->getNameInDocument());
                 auto* newLink = static_cast<App::Link*>(newObj);
                 newLink->LinkedObject.setValue(obj);
-                newLink->Label.setValue(obj->Label.getValue());
+                newLink->Label.setValue(makeInstanceLabel(doc, obj->Label.getValue()));
                 addObject(newLink);
                 link = newLink;
             }

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-*)