fix(ssr): add getServerSnapshot to useObservable's useSyncExternalStore

[tyler-reitz]

Fixes#748. v5 is not the default branch, so that keyword will not fire on merge. #748 needs hand-closing.

useObservable called useSyncExternalStore with two arguments. React requires a third, getServerSnapshot, whenever the tree is server rendered or hydrated. Without it React throws Missing getServerSnapshot, which is required for server-rendered content and the surrounding subtree silently falls back to client rendering, which is why a Next.js App Router page using any reactfire hook loses SSR for that subtree.

The part worth reviewing

The issue proposes returning "the same seeded immutableStatus the client snapshot returns". That version is unsafe and this PR deliberately does not do it.

preloadedObservables is a Map on globalThis, keyed only by observableId. In a browser that is one user's cache. On a server it is shared by every concurrent request, so a getServerSnapshot that read observable.immutableStatus would render data request A fetched into request B's HTML whenever both touch the same path.

That leak is unreachable today only because SSR throws before it can happen. So fixing the crash the obvious way would trade a crash for a cross-request data disclosure, which is the worse of the two.

This implementation reads only config, which arrives from the caller on the current render and is therefore per-request. With initialData it reports success and that value; without it, loading. The result is held in a ref so the value is stable across renders, which is what React's "The result of getSnapshot should be cached" check wants.

Verification

Four tests under a new Server rendering block, and both mutations were run rather than assumed:

• Drop the third argument: all four fail, with React's own Missing getServerSnapshot error. So the tests are load-bearing rather than decorative.
• Return observable.immutableStatus instead (the straightforward implementation): 3 of 4 pass and only the leak test fails. That is the test that earns its place, and it is why the constraint above is written down in the code rather than left to reviewer memory.

tsc passes on both tsconfig.json and tsconfig.test.json; useObservable is 22/22; eslint reports 0 errors on both changed files.

Notes

• The use-sync-external-store/shim is a red herring. Its own implementation ignores getServerSnapshot on purpose, but it resolves to React.useSyncExternalStore whenever React exposes it, so on React 18 and 19 the third argument reaches React's real implementation. Reading only the shim would suggest this fix cannot work.
• No reference docs regeneration, since no exported symbol changed.
• The globalThis cache is still a cross-request hazard for anything that does read it on a server. This PR contains the hazard at the one place it would otherwise become reachable; it does not fix the cache itself. That is separate SSR work.
• One eslint-disable for react-hooks/exhaustive-deps, with the reason in a comment above it: callers routinely pass a fresh config literal each render, and the ref means the value is computed once per component instance anyway.