Module graph 

Directed graph for focus navigation relationships.

Each node represents a focusable widget identified by a FocusId. Edges encode navigation direction (up/down/left/right/next/prev).

Invariants

1. Node IDs are unique within the graph.
2. Removing a node removes all edges incident on it (both incoming and outgoing).
3. Edges reference only nodes that exist in the graph.
4. Tab-order traversal visits nodes in ascending tab_index order, breaking ties by insertion order (via FocusId).

Complexity

Operation	Time
insert	O(1)
remove	O(E) worst case (cleans incoming edges)
connect	O(1)
navigate	O(1)
find_cycle	O(V+E)

Structs

FocusGraph

Directed graph for focus navigation.

FocusNode

A focusable node in the graph.

Enums

NavDirection

Navigation direction for focus traversal.

Type Aliases

FocusId

Unique identifier for a focusable widget.