The hmPlatforms example from the Custom Classes page is broken

[OscarMarshall]

On the current main the hmPlatforms example fails to build. Here's a repro test case that Claude came up with.

# templates/ci/modules/deadbugs/hm-forward-spawn-self-parent.nix

# Regression (PR #563): an hmPlatforms-style forward (the documented pattern at
# guides/custom-classes/#example-platform-specific-hm-classes) triggers
# "spawnNode spawn root equals its parent scope" for any host with a user.
{ denTest, ... }:
{
  flake.tests.deadbugs.hm-forward-spawn-self-parent = {

    # Minimal reproduction following the documented example structure.
    # A forward in den.aspects.tux.includes routes hm${platform} → homeManager.
    # Any host with a user applying this aspect triggers the self-parent error.
    #
    # Root cause: `resolveSourceFallback` (route.nix) is called when
    # `collectedSource == []` for a complex forward route. `scopeParent` is
    # missing the host scope's parent entry (regression in PR #563's unified
    # delivery engine), so `from` falls back to the host scope itself.
    # The spawned pipeline with `bindings = {}` finds no child binding and
    # returns `rootScopeId = from`, triggering the assertion in spawn-node.nix:59.
    test-linux-host-user-with-hmplatforms-forward = denTest (
      {
        den,
        lib,
        igloo,
        ...
      }:
      let
        hmPlatforms =
          { aspect-chain, ... }:
          den.batteries.forward {
            each = [
              "Linux"
              "Darwin"
              "Aarch64"
              "64bit"
            ];
            fromClass = platform: "hm${platform}";
            intoClass = _: "homeManager";
            intoPath = _: [ ];
            fromAspect = _: lib.head aspect-chain;
            guard = { pkgs, ... }: platform: lib.mkIf pkgs.stdenv."is${platform}";
            adaptArgs = { config, ... }: { osConfig = config; };
          };
      in
      {
        den.hosts.x86_64-linux.igloo.users.tux = { };
        den.aspects.tux = {
          includes = [ hmPlatforms ];
          hmLinux.home.packages = [ ];
        };

        expr = igloo.home-manager.users.tux.home.stateVersion;
        expected = "25.11";
      }
    );

  };
}