Add live filesystem asset mode

[tomcatzh]

Summary

• add an opt-in live-assets Cargo feature without changing the existing EmbeddedSpa API
• resolve file membership and strong ETags from fresh rust-embed metadata on every request in that mode
• document the required filesystem-backed debug configuration and preserve the cached, zero-runtime-hash production default
• exercise default and feature-enabled modes in the standard local check target

Why

Filesystem-backed rust-embed debug builds already read bytes from disk, but EmbeddedSpa cached ETags and file membership during construction. Editing or adding frontend output after startup could therefore return a stale validator or an internal error. The new feature keeps one long-lived SPA service while making its development view of disk coherent.

Developer impact

Applications can forward a local feature to embedded-spa/live-assets and run a normal debug build without rust-embed/debug-embed. The Axum fallback and handler code stay identical. Release builds continue to embed assets and should omit the development feature.

Validation

• make check
• EMBEDDED_SPA_TEST_ORIGIN_PORT=28090 EMBEDDED_SPA_TEST_NGINX_PORT=28091 make test-nginx
• cargo package --allow-dirty
• cargo +1.88.0 test --all-targets
• cargo +1.88.0 test --all-targets --all-features
• cargo test --release --all-targets --all-features

The Nginx matrix covered identity, gzip, and Brotli across eight MIME types, plus MISS, HIT, REVALIDATED, and conditional 304 behavior.