repo: harden runtime and simplify repository layout

[argahsuknesib]

Summary

• persist query lifecycle state in the registry and join all worker handles on stop
• prune stale docs and rewrite the main entry docs around the current backend flow
• remove the stale in-repo Svelte dashboard now that the maintained frontend lives in a separate repository
• delete unfinished backend stubs and dead module exports
• change Fast PR Checks to focused integration smoke coverage plus example build validation

Verification

• cargo fmt --all
• cargo test --test janus_api_integration_test --all-features
• cargo test --test http_server_integration_test --all-features
• cargo test --test stream_bus_cli_test --all-features
• cargo build --example http_client_example --all-features
• cargo test --all-features
• cargo clippy --all-targets --all-features -- -D warnings