- pub struct TapHandle(()) — zero-sized marker (Option<TapHandle> compiles
to a bool, no allocation). The live tap connection lives in the binary's
DashMap<ChannelId, TapConn>, keyed by the channel's existing ChannelId
(== wire session_id per spec §5.3).
- Channel grows pub tap: Option<TapHandle>. None until Connected, set on
Connected, cleared before state advances to Closed.
- State invariant: tap and ChannelState are tied (Connected+None=bug).
- Slice-1 backwards compatible: new field is Option<...>;
Multi-tap-per-channel is the future trigger for a TapId newtype (YAGNI).
Spec ref: 2026-06-28-slice-2-agent-tap-design.md §5.2, §6.
e0616fe42d