webui-rs 4/6: Live updates over SSE — kill the 3s client polling loop

[GeorgePearse]

Depends on #28.

The React webui watches a run by polling every 3 seconds (webui/frontend/src/context/GenesisContext.tsx:267, setInterval(refreshData, 3000)). Replace with Datastar's native model: one SSE connection per viewer, server pushes fragments when something actually changes.

Scope

• Long-lived SSE endpoint via the datastar crate's axum integration; browser opens it with data-on-load
• Server-side change detection against the Turso run store (webui-rs 7: Adopt Turso embedded as the local run store (Python writer ↔ Rust reader on one SQLite-format file) #32): poll with a watermark (max generation / max program timestamp per run) on a short interval in the server, fan out to subscribed viewers only when the watermark advances. One poller per watched run, not per connection.
• On change, push patches for: generation table rows, best-program card, stats charts (webui-rs 5/6: Server-rendered SVG charts — fitness/novelty over generations (no plotly, no chart.js) #30 fragments), lineage tree (webui-rs 6/6: Server-rendered SVG lineage tree + cutover (delete webui/frontend and backend_server.py) #31)
• Heartbeat/retry semantics so a dropped connection recovers cleanly (Datastar auto-reconnects; make sure fragment patches are idempotent so a replay after reconnect converges)
• Optional nicety: an "evolution running / idle" indicator driven by watermark staleness

Definition of done

Open a run page while an evolution is executing: new generations appear without any client-side timer, and a second browser tab shows the same updates from the same server poller.