core: document lazy-init edge case in derive and select getters

[nerdalytics]

Finding

Both createDerive (lines 247-254) and createSelect (lines 286-294) in src/index.ts have a !initialized branch in their getter functions. This branch duplicates computation logic from the effect and appears unreachable under normal usage — createEffect always runs the effect synchronously.

The branch is reachable when derive() or select() is called inside batch(), because batch defers effect creation. Without the lazy-init fallback, reading a derived value inside batch would return stale data.

The code is correct. The reason it exists is not documented.

Proposed fix

Add a brief comment above each !initialized branch explaining the batch-deferred edge case. Something like:

// When derive() is called inside batch(), effect creation is deferred.
// This fallback ensures the first read still returns the computed value.

No behavioral change needed.

Source: out/report-simplify.txt, Findings 3 + 4

[nerdalytics]

#109 got rid of the internal createState backing derive and select. That was the whole reason this edge case existed. First read would call valueState.set() and spin up notification for zero subscribers. Since there's no valueState anymore, the behavior this issue wanted to document is just gone.