optional-engine-feature-degrade-in-tiers

An optional build-time dependency of a media engine may be missing from another platform's bundle, and the engine rejects the WHOLE chain string when one stage in it is unknown — so retry without the optional stage rather than losing the mandatory one, and return which tiers were accepted so callers never drive a stage that is not there. Use when a feature works on one platform's bundle and silently does nothing on…