Pure element descriptors for agent-doc document components.
Element crates describe component semantics only: marker names, aliases, ownership, realtime policy, and scheduling role. They do not read or write files, schedule turns, open IPC, invoke tmux, mutate snapshots, or load plugins. Runtime crates consume these descriptors.
Architecture: every supported element may own a small pure realtime model for its local state and reconciliation rules. The document model composes those element models and owns cross-element invariants such as backlog to queue projection, active-head marker placement, and signals that observe queue/turn/supervisor state.