Adds `memory-serve` 2.1 as an optional workspace dependency, a `build.rs`
that runs `load_directory` only when `CARGO_FEATURE_EMBED_WEB` is set, a
`web_assets` module serving `web/dist` at `/` with SPA fallback (200 OK)
for unknown client-side routes, and a feature-gated integration test.
The default build (no feature) compiles and tests cleanly without `web/dist`.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
