imports: retire the is_pub sketch on NamespaceTarget

Every namespace alias is module surface under the carry rule — the
planned pub-import front-end form is superseded; no per-edge visibility
flag is needed.

src/imports.test.zig

@@ -365,7 +365,6 @@ test "buildImportFacts: namespaced file import captures target_module_path" {
     try std.testing.expectEqualStrings(main_path, g.importer_source);
     try std.testing.expectEqualStrings(point_path, g.target_module_path);
     try std.testing.expect(g.own_decls.len >= 1);
-    try std.testing.expect(!g.is_pub);
 
     // The alias is also a `.namespace_decl` in the importer's scalar index.
     const m_idx = facts.decls.get(main_path) orelse return error.MissingMainIndex;