Concurrent or nested delegation batches (a parent's 9-way fan-out plus a
child's own 3-way fan-out) printed interleaved `✓ [3/3]` / `✓ [3/9]` lines
with nothing identifying which batch each belongs to.
- CLI: batch header `🔀 [6a66] delegating 9 tasks`; completion lines and
child tree-view lines become `[6a66 3/9]`; spinner remaining-count tagged.
- Relay: `delegation_id` rides on every `subagent.*` event (TUI gateway
payload, api_server SSE subagent.start/complete).
- TUI: `[6a66 3/9]` prefix on /agents rows; Desktop Agents pane groups
workers by exact delegation_id (heuristic shape/time grouping kept for
older backends) and shows the tag on the group header.
- Tag = last 4 hex of the deleg_xxxxxxxx id (format_batch_tag), same id
returned by the dispatch and used for cache/delegation/live/<id>/.
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