standalone: LibsSource abstraction (remote over the contract + static fallback),
generic window.kicadLibs provider, sym-lib-table generated from the backend's
lib list at boot (replaces the spike). backend: serve pre-built self-contained
symbol bodies from LIBS_DIR (listLibs/listLibItems + raw item-body route).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>