fix(benchmark): allow baseline scenarios with no overlays

[Tehsmash]

Summary

Fixes a bug where benchmark scenarios declared with empty overlays (overlays: {}) were silently skipped, making it impossible to run an app "baseline" scenario with no configuration changes.

Root causes:

1. OverlayStack.from_dict injected logic=[scenario_id] whenever all overlay lists were empty, defeating any explicit attempt to declare a no-overlay scenario. The default should only fire when the overlays key is absent entirely — which still happens via MASScenarioSpec.from_dict.

2. The base-manifest loading path in preload_scenario_configs was gated on configs_dir is None, so it was unreachable in experiments mixing overlay and no-overlay scenarios.

Fix:

• Remove the fallback injection in OverlayStack.from_dict
• Extend the base-manifest path to also trigger when the scenario's flattened overlay list is empty

A baseline scenario can now be declared as:

scenarios:
  - id: baseline
    overlays: {}