add some guard rails w.r.t. padding

src/user.zig

@@ -199,8 +199,19 @@ pub const UserReader = struct {
 
 const testing = std.testing;
 
-test "PackedUser is byte-aligned" {
+test "PackedUser alignment" {
+    // byte-aligned
     try testing.expectEqual(0, @rem(@bitSizeOf(PackedUser), 8));
+
+    const bytes = @divExact(@bitSizeOf(PackedUser), 8);
+
+    // External padding (PackedUserAlignmentBits) must be higher or equal to
+    // the "internal" PackedUser alignment. By aligning PackedUser we are also
+    // working around https://github.com/ziglang/zig/issues/10958 ; PackedUser
+    // cannot be converted from/to [@bitSizeOf(PackedUser)/8]u8;
+    // asBytes/bytesAsValue use @sizeOf, which is larger. Now we are putting no
+    // more than 1, but it probably could be higher.
+    try testing.expect(@sizeOf(PackedUser) - bytes <= 1);
 }
 
 fn testShellIndex(shell: []const u8) ?u6 {