feat(gui): add document-origin tracking and display in window title (#16)

- Add originForDocument(), setDocumentOrigin(), clearDocumentOrigin()
  methods to OriginManager
- Add signalDocumentOriginChanged signal for UI updates
- Add document-to-origin tracking map in OriginManager
- Update MDIView::buildWindowTitle() to append origin suffix for
  non-local origins (e.g., 'Part001 [Silo]')

This implements Issue #16: Document origin tracking and display

src/Gui/OriginManager.cpp

@@ -231,6 +231,63 @@ FileOrigin* OriginManager::findOwningOrigin(App::Document* doc) const
     return nullptr;
 }
 
+FileOrigin* OriginManager::originForDocument(App::Document* doc) const
+{
+    if (!doc) {
+        return nullptr;
+    }
+
+    // Check explicit association first
+    auto it = _documentOrigins.find(doc);
+    if (it != _documentOrigins.end()) {
+        FileOrigin* origin = getOrigin(it->second);
+        if (origin) {
+            return origin;
+        }
+        // Origin was unregistered, clear stale association
+        _documentOrigins.erase(it);
+    }
+
+    // Fall back to ownership detection
+    FileOrigin* owner = findOwningOrigin(doc);
+    if (owner) {
+        // Cache the result
+        _documentOrigins[doc] = owner->id();
+        return owner;
+    }
+
+    return nullptr;
+}
+
+void OriginManager::setDocumentOrigin(App::Document* doc, FileOrigin* origin)
+{
+    if (!doc) {
+        return;
+    }
+
+    std::string originId = origin ? origin->id() : "";
+
+    if (origin) {
+        _documentOrigins[doc] = originId;
+    } else {
+        _documentOrigins.erase(doc);
+    }
+
+    signalDocumentOriginChanged(doc, originId);
+}
+
+void OriginManager::clearDocumentOrigin(App::Document* doc)
+{
+    if (!doc) {
+        return;
+    }
+
+    auto it = _documentOrigins.find(doc);
+    if (it != _documentOrigins.end()) {
+        _documentOrigins.erase(it);
+    }
+}
+
 FileOrigin* OriginManager::originForNewDocument() const
 {
     return currentOrigin();