Conditional UI and keyed lists API

All operators need a stable owner and return Source<TResult[]>. Treat result arrays and callback signals as operator-owned state. Bind results with Children(() => output.Value).

Operator	Identity and constructor inputs
Show(condition, build, fallback = null)	Boolean branch; advanced constructors receive presence.
Show(read, truthy, build, fallback = null)	Boolean branch retaining the last truthy value; constructor receives filtered value and presence.
Switch(select, build, comparer = null)	Selected key; advanced constructor receives fixed key and presence.
Switch(select, branches, fallback = null)	Dictionary of constructors receiving presence. Unmatched keys without fallback produce the default result.
Indexes(keyedGetter, build, comparer = null)	Fixed key, reactive value and presence. Input is an enumerable of key/value pairs.
Indexes(listGetter, build)	Fixed zero-based list position, reactive value and optional presence.
Values(listGetter, build, comparer = null)	Fixed value, reactive zero-based index and optional presence.

Advanced constructors return Branch<TResult>(result, exitDelaySeconds = 0); a plain result converts implicitly to a zero-delay branch. Convenience overloads return TResult directly: Show(condition, () => result), Switch(select, key => result), positional Indexes(read, (index,value) => result) and Values(read, (value,index) => result).

var visible = UI.Source(true);
var output = UI.Show(() => visible.Value, () => UI.Label("Hello"));
var host = UI.Column(UI.Children(() => output.Value));

Retained exits

When a row departs, presence becomes false. A zero-delay branch is removed immediately. A delayed branch remains in the output until its timer runs; reentry before expiry cancels removal and reuses its scope and result. Departing Values rows receive index -1.

Active rows follow input order, then retained exiting rows. Output arrays publish only when membership/order changes. Retained row value/index signals can change without publishing another result array.

Keys must be unique and non-null; Values requires unique non-null identities as well. Default identity uses EqualityComparer<T>.Default; supply a comparer for a different policy. List Indexes keys are indices, not item identities. These operators reconcile retained UI by identity.

Immediate removals attempt other removed-row cleanups after one fails. Both immediate and delayed removals publish structural removal despite cleanup failure, then report errors through the scheduler/clock. Disposing the owner attempts all row cleanups. A constructor failure disposes the operator rather than providing transactional rollback.

Exit delays must be finite and non-negative. Automatic unscaled time advances timers; after UI.Step, keep advancing the manual clock yourself.