v1.5.1

Smoothing fixes, trajectory graphics, and a new flux module.

• smoothing: Savitzky-Golay track ends are evaluated on the polynomial fitted to the first/last window (no edge bias); tracks with frame gaps are smoothed on a uniform time grid; new min_window keeps short tracks (smoothed with a shorter window) instead of dropping them.
• graphics: interactive PyVista 3D trajectory plot (plot_trajectories_3d); trajectory_summary and track selections -- longest, fastest, path (travelled distance), typical, coverage, jumps; scripts/view_trajectories_3d.py.
• flux (new, pip install flowtracks[flux] for the centreline): fluid domain from particle positions, centreline and cross-sections, section lumen, particle-based volume flow rate per section and time window with coverage, flow rate of a known velocity field, and the domain's outward boundary faces.
• ruff config and CI gate.