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fix(web,compiler,babel-plugin): a lone spread passes its accessor through — no merge (#3105)

Merging one source is pure overhead, and the mergeProps memo consumed a
hydration id the SSR fast path never allocated, so every element after a
reactive lone spread went unclaimed. Both DOM generates now hand the
accessor straight to spread(), which resolves a function props source
inside its own tracking scopes (the Solid 1 shape). Server output is
untouched; the ids agree because neither side mints anything.

Supersedes the PR's server-side approach (deferring a merge until after
the element key): the client is the side doing unnecessary work.

The universal generate keeps its merge: it has no hydration ids to
drift, and its condition-memo insulation for arbitrarily expensive
custom-renderer props is a documented trade-off.

New hydration-parity harness scenario pins consecutive ids across the
spread (div _hk=0, button _hk=1) end to end.

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

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