Refuse base-repo installs under GL_SCOPE_USR at the library level

gl_install_pkg now returns GL_INSTALL_ERR_SCOPE for
scope == GL_SCOPE_USR && repo == "base", checked immediately after
manifest parse and before any store path construction or filesystem
mutation. Per SECURITY_DESIGN.txt section 8: enforced in the library
rather than a CLI, since the split-.so architecture lets third-party
tools call gl_install_pkg directly.

Also drops src/istoreutils.c.rej, .c.back, .h.back — stale leftovers
from the uid -> gl_context_t migration patch; that migration is fully
present in the current tree, so these were dead weight.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/istoreutils.c

@@ -1188,6 +1188,27 @@ gl_install_pkg(gl_context_t *ctx, const char *gpkg_path)
 	if (!manifest_found) {
 		return GL_INSTALL_ERR_MANIFEST;
 	}
+
+	/*
+	 * Base-repo packages can NEVER be installed under per-user scope.
+	 * Enforced here, at the library level, rather than in any CLI: the
+	 * split-.so architecture means a third-party tool can call
+	 * gl_install_pkg directly without going through gpkg/syspkg, and a
+	 * rule only one CLI respects is not a real guarantee. See
+	 * SECURITY_DESIGN.txt section 8.
+	 *
+	 * This is the earliest possible point — pkg.pkg_repo only exists
+	 * once the manifest is parsed — and deliberately sits before any
+	 * store path construction or filesystem mutation below.
+	 */
+	if (ctx->scope == GL_SCOPE_USR &&
+	    strcmp(pkg.pkg_repo, GL_BASE_REPO) == 0) {
+		lg_printf(2,
+			"Refusing to install base-repo package '%s' under user scope.",
+			pkg.pkg_name);
+		gl_free_pkg(&pkg);
+		return GL_INSTALL_ERR_SCOPE;
+	}
 	
 	char pkg_store_base[PATH_MAX];
 	char pkg_store_final[PATH_MAX];