Slot:Add now accepts State<T>/Source<T> elements directly, arbitrarily
nested. Implemented as pure sugar (isState(element) -> internally
Slot():Single(element)) rather than a new position-keyed StoreBind
mechanism, since the latter would reintroduce array-part None, Length
special-casing, and Move/Swap index-subscription sync bugs that :List's
key-based design specifically avoids. Slot:Single's updateFn is now
optional (defaults to identity) to support this sugar. Also fixes
:List's reconcile to skip the compact index by a nested-Slot result's
.Length instead of a flat +1, so multi-root list items don't produce
overlapping LayoutOrder ranges for subsequent siblings.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ymPLYAQmEYnTrnrdAwxpU
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