Pull duplicated rules into ui-tui/src/lib/subagentTree so the live overlay,
disk snapshot label, and diff pane all speak one dialect:
- export fmtDuration(seconds) — was a private helper in subagentTree;
agentsOverlay's local secLabel/fmtDur/fmtElapsedLabel now wrap the same
core (with UI-only empty-string policy).
- export topLevelSubagents(items) — matches buildSubagentTree's orphan
semantics (no parent OR parent not in snapshot). Replaces three hand-
rolled copies across createGatewayEventHandler (disk label), agentsOverlay
DiffPane, and prior inline filters.
Also collapse agentsOverlay boilerplate:
- replace IIFE title + inner `delta` helper with straight expressions;
- introduce module-level diffMetricLine for replay-diff rows;
- tighten OverlayScrollbar (single thumbColor expression, vBar/thumbBody).
Adds unit coverage for the new exports (fmtDuration + topLevelSubagents).
No behaviour change; 221 tests pass.
5b0741e986