chore: refactor retrieval pipeline to chunk-first RRF with derived entities and slimmer eval surface.

Collapse the multi-strategy entity engine into one benchmarked chunk retrieval path, derive entities from retrieved chunks, and update consumers, docs, and clippy fixes across the workspace.

main/src/main.rs

@@ -75,6 +75,7 @@ struct AppState {
 }
 
 #[cfg(test)]
+#[allow(clippy::expect_used)]
 mod tests {
     use super::*;
     use axum::{