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perf(web): spread() creates fewer reactive nodes and accepts a sources array (#3388) (#3419)

`ref` folds into the attribute effect: it is collected with the other props
in the compute half and re-applied in the commit half only when its identity
differs from the last applied one. `ref()` runs the callback untracked with
no owner, so nothing a ref creates is disposed when the effect reruns; that
is what makes the fold safe.

Children keep their own owned `insert`. That effect owns the child subtree
(components, memos and effects created while the children getter runs are
disposed when it reruns), so merging it into the attribute effect would tear
the children down and rebuild them on every attribute change. A plain object
whose `children` is a data property inserts the value with no effect at all.
Three nodes become two when children flow through the spread, one when they
don't.

`spread(el, [a, b], skipChildren, skip)` takes an array of sources: the
union of own string keys with later sources winning, only the winning source
read, function sources called inline with no memo and so no hydration id,
`skip(key)` never read nor applied. Same contract as the server `ssrElement`
array form. Web effects are transparent for hydration ids, so dropping the
ref effect shifts no sibling ids.

Co-authored-by: Cursor <cursoragent@cursor.com>

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