Compute props through the data store

[pleek91]

Description

Second step of #805, stacked on #806. No behaviour changes — every existing test passes unchanged.

Props move onto the store added in #806, one store per navigation. A link prefetches into its own store and stages it when followed; the navigation it triggers adopts that store, and the one it replaces is disposed outright rather than picked over for entries the new route still needs.

A getter waiting on its parent now awaits a value rather than watching for one to appear:

await Promise.any([
  own.subscribe(parentKey),                  // the parent's props, if this link prefetches them
  navigation.current().subscribe(parentKey), // or the ones navigation computes
])

That removes the waiter machinery entirely — the detached effect scope, the watch handles, and the bookkeeping to discard them. Prefetch strategies also stop being separate buckets, so a value prefetched on render is visible to a getter that runs on hover without being passed along explicitly.

[stackoverfloweth]

left some small suggestions in a commit, love how much this simplifies the prefetching. The props store changes are pretty sweeping and hard to really grok fully but I like what I see