readonly-stateflow-cold-to-hot

Expose state through asStateFlow() rather than an upcast of the mutable holder, and turn a cold source flow hot with stateIn(scope, WhileSubscribed(timeout), initial) so it survives collector churn without running forever. Use when a screen re-queries its source on every rotation or navigation, when work continues after the last collector leaves, or when something outside the owner is writing state it should only be…