Add in-process ring buffer history for per-worker and per-actor stats,
with sparkline rendering on both web (SVG) and TUI (ratatui) surfaces.
- history.rs: DashboardHistory with bounded VecDeque ring buffers, LRU
actor eviction, thread-safe RwLock, 4 unit tests
- SSE producer records stats every tick, sends initial history snapshot
- /api/history REST endpoint for on-demand history JSON
- Web: inline SVG sparklines in worker detail headers (rates + mailbox)
- TUI: per-worker Sparkline widgets in overview layout
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
144b4ca5ef