gen-config — typed operator-facing configuration for the gen
engine. Implements [shikumi::TieredConfig]; every consumer
resolves via GenConfig::resolve_from_env("GEN_TIER") at startup.
The four typed slot groups (workspace / cache / render / target) cover every knob an operator can flip without recompiling. New knobs land here, not in CLI flags — flags are a view, config is the substrate.