feat: only traverse trailing static nodes during hydration

[Rich-Harris]

Alternative to #12932 — we continue traversing if hydrating (which is annoying, but allows the mismatch detection to continue working) but not if mounting. If the app never hydrates, the $.next(...) line will be treeshaken.

If we find an elegant way to skip traversal in hydration in cases like #12932 then that would be even better, but this is a useful improvement in its own right.

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[trueadm]

Looks good to me.