feat(library): page Artists from local catalog

Materialize source-scoped artist aggregates and FTS, then expose exact keyset pages and bounded A-Z metadata for the existing Artists rail.

Query the selected artist's albums through the indexed credit relationship with an independent bounded model, generation cancellation, artwork updates, and per-surface legacy fallback.

cx

crates/qbz-qt/build.rs

@@ -304,6 +304,7 @@ fn build_rhi_items() {
         "tunnelflow_item",
         "local_tracks_model",
         "local_albums_model",
+        "local_artists_model",
     ] {
         let header = format!("cxx/{item}.h");
         let source = format!("cxx/{item}.cpp");