Empty Group in display_item_urgency and display_item_line has no assertion — silently returns Low urgency

[ooloth]

Why

display_item_urgency returns Urgency::Low as the default when a Group has zero items. The current code prevents this path, but there is no assertion that a Group always contains at least one item — a future refactor that constructs a stale DisplayItem::Group { items: vec![] } would silently be assigned Low urgency with no diagnostic.

Current state

ui/tui/src/display.rs lines 337–344: DisplayItem::Group { items, .. } => items.first().map(item_urgency).unwrap_or(domain::Urgency::Low). The unwrap_or fallback is currently unreachable but is not guarded by an assertion. A caller holding a zero-item Group silently receives the lowest possible urgency.

Ideal state

• An assertion assert!(!items.is_empty(), "Group must contain at least one item") is present in display_item_urgency (and symmetrically in display_item_line).
• The empty-group invariant is explicit rather than implicitly maintained by construction order.

Starting points

• ui/tui/src/display.rs — lines 337–344 (display_item_urgency) and the corresponding arm in display_item_line

QA plan

1. Construct a DisplayItem::Group { items: vec![] } and call display_item_urgency on it — expect an assertion failure rather than Urgency::Low.
2. Run cargo test — expect all existing tests pass.

Done when

display_item_urgency asserts a non-empty items list for Group variants, making the empty-group invariant explicit and catchable.