fix(split): keep per-volume file-window state stable during volume cycling

[robkam]

Summary

• preserve per-volume file-window state across volume cycling in split workflows
• stop clearing per-directory big_window state during cycle transitions
• persist/restore saved big-file-window state with each panel+volume snapshot
• include maintainer-edited tracker updates in docs/ROADMAP.md and docs/BUGS.md

Validation

• make clean && make
• pytest -q tests/test_panel_isolation.py -k "volume_cycle and (window_mode or leak_state or split_file_selection_preserves_panel_local_volume_cycle_state)"
• pytest -q tests/test_panel_isolation.py

Notes

• draft PR opened early; full-QA CI gate remains required before merge