During a token stream $messages is replaced ~30x/s. Subscribing the whole
chat view to it re-rendered the composer, runtime boundary, and every
message on every delta.
- Derive coarse facts (empty thread? tail is user?) via nanostores
`computed` atoms so per-token flushes don't re-render their consumers.
- Move the $messages subscription + runtime wiring into a dedicated
ChatRuntimeBoundary; the composer reads $messages imperatively.
- Drive message rows off stable useAuiState selectors and a lazy
getMessageText getter instead of eagerly materialized text.
- Feed ResizeObserver entry sizes into measureClamp / FadeText and dedupe
the style writes, killing the read-write-read reflow cascade.
Brooklyn Nicholson
7c226cc57f