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feat(web): ssrElement accepts an array of prop sources and a skip predicate (#3418)

`ssrElement(tag, [a, b, c], children, needsId, skip)` serializes straight
from the sources with the exact output of `ssrElement(tag, merge(a, b, c),
...)`: later sources win per key, attributes land in merged order, only the
winning source's getter is read (once), and `skip(key)` drops a key from
every source without reading it. Libraries and compilers spreading several
sources onto an element no longer build an intermediate merged object that
is walked once and discarded.

The array — or a thunk yielding it — is resolved after the hydration key is
taken, so a source getter that renders a child cannot shift the element's
own id. A function source is a plain thunk called once: no memo, no
hydration ids, matching the client `spread` array form. Nullish sources are
empty. The single-object and thunk forms are unchanged.

Measured on the yak-bench SSR suite with next-yak's runtime calling this
instead of its hand-written serializer: elements with dynamic props go
from 2.1-2.9x slower than the hand-written path to 1.1-1.2x; suite geomean
2.43x -> 1.69x.

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

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