feat(library): bootstrap derived catalog

Build the catalog side by side from read-only local, Plex, Jellyfin, and Subsonic cache snapshots after the first useful Qt frame.

Persist bounded batch checkpoints, preflight disk capacity, validate materializations, and activate generations atomically while preserving the existing reader as fallback.

cx

crates/qbz-qt/Cargo.toml

@@ -75,6 +75,9 @@ qbz-models = { path = "../qbz-models" }
 qbz-cache = { path = "../qbz-cache" }
 qbz-offline-cache = { path = "../qbz-offline-cache" }
 qbz-library = { path = "../qbz-library" }
+# Derived, rebuildable Local Library read catalog. The Qt app only schedules
+# its background bootstrap here; no Qt type crosses into the catalog crate.
+qbz-local-catalog = { path = "../qbz-local-catalog" }
 # MyQBZ: Mixtapes & Collections backend (schema + migrations, repo, shuffle,
 # enqueue). Headless and frontend-agnostic (ADR-006) — the same crate the
 # Slint app uses (crates/qbz/Cargo.toml:96), so the two frontends share