refactor(qbz-qt): split QbzLibrary out of the God-object

library_bridge.rs has sat in the tree unreferenced since the phase-23 work
stopped: no mod declaration, not in build.rs, zero call sites, and duplicating
members that were still live in bridge.rs. It could not have been wired as it
stood — its module was named qbz_library, which is ambiguous with the
qbz-library crate this binary depends on, so it never compiled. The module is
now qbz_library_bridge; the QML type name comes from the QObject, so QML is
unaffected.

Registering it and deleting the duplicates had to happen together. Wiring it
alone would have compiled cleanly and produced two independent Library states —
QML reading one object while Rust published to the other — with the view simply
frozen and nothing in the log.

Moved with it: four properties, five invokables, three signals, the seven
library-only ui() hops in main.rs, and thirty-four QML references. The
Connections blocks in LibraryView, Sidebar, QueuePanel, AlbumView and
PlaylistCollage retarget wholesale because every handler in them is a library
signal. ArtistView's does not: it mixes onLibraryArtworkReady with
onReleaseSectionReady, so it becomes two blocks — retargeting it whole would
have orphaned the releases pager, and QML resolves handlers lazily enough that
the discography would just have stopped loading with nothing in the log.

bridge.rs is down to 389 lines and five domains.

cc

crates/qbz-qt/qml/shell/QueuePanel.qml

@@ -42,7 +42,7 @@ Rectangle {
     QbzTheme { id: theme }
 
     Connections {
-        target: QbzBridge
+        target: QbzLibrary
         function onLibraryArtworkReady(key, path) {
             var m = root.coverMap
             m[key] = path