v0.1.3 -- real mesh arm + articulated GN01 gripper in the live mirror

The live mirror now renders the actual Rizon 4s mesh and a fully articulating GN01 gripper. flexiv_description ships only xacro sources, so the [viz] extra now generates a self-contained URDF (standalone xacro, no ROS) with two fixes the vendor sources need outside a ROS workspace: mesh paths absolutized (package:// and package-relative both break) and nested mimic chains flattened (yourdfpy resolves one level; the GN01 4-bar chains two -- five of six finger joints froze). Joints map by name; the gripper articulates straight from the streamed width. Run: flexiv-control viz --connect --fetch-assets.