refactor(vis): H4.5 steps 4-5 — statistics and the three reader gates on the canonical resolver; core goes package-level

VisibleLibrarySet becomes a package-level function taking the queries
(db-only services like statistics reach it without new wiring); the
LibraryService method stays as the ergonomic entry point. Statistics'
ids-shaper and the reader pages' three hand-rolled membership loops
(empty-rows fallback then linear scans) now delegate; the gates keep
their 403-on-invisible semantics (>null set membership, never silent
pass).

Census addendum from this pass: two OPDS sites call
GetUserVisibleLibraries directly but consume FULL ROWS (per-library
folder enumeration) — a third legitimate pattern alongside set/ids and
resolveLibrary's active-selection; documented, untouched. The library
LIST endpoint and its SSR data helper are row consumers too, by design.

internal/services/library_service.go

@@ -174,7 +174,8 @@ func (s *LibraryService) SetLibraryVisibility(ctx context.Context, userID, libra
 	return &visibility, nil
 }
 
-// GetUserVisibleLibraries retrieves all libraries visible to a user
+// GetUserVisibleLibraries retrieves all libraries visible to a user.
+
 // VisibleLibrarySet is THE canonical library-visibility resolver (H4.5):
 // every endpoint that asks "which libraries can this user see" funnels
 // here. The family-lens rule itself lives in the
@@ -182,8 +183,8 @@ func (s *LibraryService) SetLibraryVisibility(ctx context.Context, userID, libra
 // default-allow: an admin hides a library, everyone else still sees it).
 // Callers adapt their own identity source (JWT user object, device
 // token, SSR session); this core takes the resolved user id.
-func (s *LibraryService) VisibleLibrarySet(ctx context.Context, userID pgtype.UUID) (map[string]bool, []pgtype.UUID, error) {
-	rows, err := s.GetUserVisibleLibraries(ctx, userID)
+func VisibleLibrarySet(ctx context.Context, db *database.Queries, userID pgtype.UUID) (map[string]bool, []pgtype.UUID, error) {
+	rows, err := db.GetUserVisibleLibraries(ctx, userID)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to check library access: %w", err)
 	}
@@ -199,6 +200,12 @@ func (s *LibraryService) VisibleLibrarySet(ctx context.Context, userID pgtype.UU
 	return set, ids, nil
 }
 
+// VisibleLibrarySet is the LibraryService entry point for the
+// package-level core (see the core's doc).
+func (s *LibraryService) VisibleLibrarySet(ctx context.Context, userID pgtype.UUID) (map[string]bool, []pgtype.UUID, error) {
+	return VisibleLibrarySet(ctx, s.db, userID)
+}
+
 func (s *LibraryService) GetUserVisibleLibraries(ctx context.Context, userID pgtype.UUID) ([]database.GetUserVisibleLibrariesRow, error) {
 	return s.db.GetUserVisibleLibraries(ctx, userID)
 }