Module hierarchy 

Task hierarchy indexing and navigation based on parent_id.

Responsibilities:

• Index tasks by parent_id for efficient child lookups.
• Provide safe traversal of parent/child relationships with cycle detection.
• Treat missing-parent tasks as “roots” and mark them as orphans during rendering.

Not handled here:

• Queue persistence (see crate::queue).
• Validation logic (see crate::queue::validation).

Invariants/assumptions:

• Task IDs are unique across active + done files.
• Ordering is deterministic: active tasks first (file order), then done tasks (file order).
• Cycles are detected and reported but do not cause infinite recursion.