webui-rs 1/6: Scaffold Rust + Datastar webui service (axum + datastar crate)

[GeorgePearse]

Part of the Rust + Datastar webui rewrite (replaces the FastAPI + React 19/Vite/d3/plotly stack — decision 2026-08-05: full Datastar, server-rendered SVG, no d3/React islands, and the backend is Rust, not FastAPI).

Scope

• New crate (suggest webui-rs/ at repo root, or a cargo workspace if we expect more Rust) using:
  • axum + datastar (official SDK, has axum integration: SSE stream wrappers + signals extractor)
  • Storage: Turso embedded (turso crate, in-process) per webui-rs 7: Adopt Turso embedded as the local run store (Python writer ↔ Rust reader on one SQLite-format file) #32 — the webui reads the run file directly, no DB server. (Original plan was the clickhouse crate mirroring the FastAPI backend; superseded by webui-rs 7: Adopt Turso embedded as the local run store (Python writer ↔ Rust reader on one SQLite-format file) #32, which demotes ClickHouse to an optional sink.)
• Serve the app shell: one HTML page, Datastar script inlined/vendored (no CDN dependency, no node_modules, no build step)
• Config parity with genesis/webui/backend_server.py: ClickHouse URL from the same env var(s), same defaults; /healthz reports whether the run store opened (mirrors the current health field, minus the ClickHouse server dependency)
• cargo run serves on the same port the FastAPI backend uses today so docs/muscle memory keep working

Non-goals here

Data endpoints, SSE push, SVG rendering — issues 2–5. This is just the skeleton that compiles, connects, and serves the shell.