Add a snapshot children_by_id view for tree containers

[Flerpharos]

Summary
Add a stable point-in-time child-by-ID snapshot on tree containers.

Scope

• provide a read-oriented mapping keyed by child ID
• make snapshot semantics explicit rather than exposing mutable internal state
• document how the snapshot behaves across later refreshes and mutations

Acceptance criteria

• callers can get a stable mapping keyed by child ID
• the snapshot does not mutate when the container later changes
• docs describe snapshot semantics clearly