Add radius attribute for rounded corners on orthogonal edges

Implements rounded corner support for orthogonal edge routing with opt-in behavior to maintain backward compatibility.

Features:

• Supports radius=N attribute for explicit radius control eg. edge [radius=5];
• Detects orthogonal corners by analyzing direction changes
• Truncates edge segments at corners by the radius distance
• Supports mixed rounded and square corners in same graph eg. A1 -> B3 [color=red,radius=12];

Backward compatibility:

• Radius defaults to 0 (square corners) maintaining existing behavior
• Only applies rounding when radius > 0 is set
• No behavior change for existing graphs without rounded style