Initialise the rotation of a new line and polygon in the element editor

PartLine and PartPolygon keep their rotation in m_rot, which nothing
sets until setRotation() is called. rotation() reads it, and
setRotation() computes the difference from it, so the first rotate of
a freshly drawn line or polygon in the element editor starts from
garbage: qRound(m_rot * 100.0) on an arbitrary value is undefined
behaviour, and the angle stored afterwards is arbitrary too.
PartRectangle and PartEllipse already start at 0.

Give m_rot a default value of 0 in both headers, as PartEllipse has.
Only the two headers change.

No file-format change, and no change for a part loaded from a file or
rotated once, since both set m_rot before reading it.

Tests: none of their own; an uninitialised read is what a sanitizer
finds, and this one is the same kind as the BorderTitleBlock and
ConductorProfile fixes (#1358, #1359). ctest on macOS, and all
example projects through --resave and --export-svg, unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: Beat Hangartner <beat@hangartners.ch>

sources/editor/graphicspart/partline.h

@@ -127,6 +127,6 @@ class PartLine : public CustomElementGraphicPart
 		int m_vector_index = -1;
 		QPropertyUndoCommand *m_undo_command;
 		QVector<QetGraphicsHandlerItem *> m_handler_vector;
-		qreal m_rot;
+		qreal m_rot = 0;
 };
 #endif

sources/editor/graphicspart/partpolygon.h

@@ -119,6 +119,6 @@ class PartPolygon : public CustomElementGraphicPart
 		QAction *m_insert_point,
 				*m_remove_point;
 		QPointF m_context_menu_pos;
-		qreal m_rot;
+		qreal m_rot = 0;
 };
 #endif