fix: make prerender cache work, including in dev

[Rich-Harris]

The idea is that we store prerendered data in an origin-scoped cache automatically, and consult that cache before going to the network. (Eventually, we'll do the same for cacheable query data as well.)

It doesn't work at the moment. This fixes it. It also adds a prerender cache in dev, which survives until a page reload (not longer, since sticky caches in development are annoying).

No test because I'm at a conference and don't have time right now; I think we could add one in a follow-up if nec.

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