Adds two startup-time env toggles that Phase D.2b's retail-UI panel
frame will read:
- ACDREAM_RETAIL_UI=1 → opts.RetailUi (bool, default false)
- ACDREAM_AC_DIR=<path> → opts.AcDir (string?, default null)
Both follow the existing helper conventions (IsExactlyOne / NullIfEmpty).
No call sites broke because the only construction site in RuntimeOptions.cs
already uses named arguments.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
