Multi-file trajectory paths not handled during save/load relativization #1113
Description
MDAnalysis supports universes created with multiple coordinate files (e.g. mda.Universe(top, [traj1.xtc, traj2.xtc, traj3.xtc])). The current path relativization in session.py only converts traj.universe.trajectory.filename (singular), so only one file path gets relativized during save. Any additional trajectory files would retain absolute paths and break when the folder is moved.
This isn't an issue for the GUI today since it only supports loading a single trajectory file, but it affects API users who create multi-file universes.
Expected behavior
All coordinate file paths associated with a trajectory should be relativized on save and restored to absolute on load.
Context
Flagged by @PardhavMaradani during the #1088 discussion. Low priority since the GUI doesn't expose multi-file trajectories, but worth tracking for API completeness.