Sort idempotency: add UUID as first (least significant) sort pass to
guarantee deterministic ordering for items with equal sort keys.
Without this, pairs with identical subjects flip-flop on every re-sort
due to the missing statusSortFunction silently falling back to subject
(creating conflicting forward/reversed subject passes).  Log the
silent fallback so missing sort functions are visible.

Skip redundant rebuilds: before deleting and recreating all tree nodes,
snapshot the current tree structure (object IDs in depth-first order)
and compare against what the presentation would produce.  If identical,
refresh items in place (text, colors, font) without rebuilding.

Remove save freeze/rebuild: viewers no longer subscribe to
aboutToSave/justSaved - saving to disk does not change the
presentation.

Narrow EventFilter to motion events only: clicks, keyboard, and scroll
pass through so the app stays responsive during rebuilds.

PEP 8 renames across sorter, viewer, container, frame, itemctrl, and
all callers: snake_case for methods, locals, and parameters.  Renamed
~80 identifiers across 22 files including sort_by, sort_keys,
sort_ascending, sort_case_sensitive, sort_event_type, is_ascending,
create_sort_key_function, add_pane, set_pane_title, docked_panes,
is_center_pane, bind_events, active_viewer, activate_viewer,
add_viewer, close_viewer, advance_selection, components_created,
is_showing_tasks/effort/notes/attachments/categories, on_begin_io,
on_end_io, register_presentation_observers, send_viewer_status_event,
on_presentation_changed, init_layout, set_title, show_sort_column,
show_sort_order, expand_all, collapse_all, hide_column,
check_all_categories, get_is_item_checked, on_item_popup_menu,
on_drop_url/files/mail, on_column_click, on_end_column_resize.