Determines whether a cached memo entry is still valid.

The validation algorithm recursively checks dependencies to avoid recomputation when nothing has changed. Early cutoff is integrated: even after a dependency re-executes, if its value hasn't changed (changed_at stays old), downstream queries remain valid.

The algorithm

Four cases, checked in order:

1. No memo — the query has never been computed. Return :stale.
2. Already validated — verified_at == current_rev. Return :valid.

• Code version — the entry was computed by another version of its query's code than the registered one (Roux.Query). Return :stale: it re-executes, and early cutoff keeps its changed_at when the value comes back the same.

3. Durability skip — no input at this query's durability level (or below) has changed since verified_at. Update verified_at and return :valid without walking dependencies.
4. Walk dependencies — for each dependency, call ensure_fn to bring it up to date, then check its changed_at. First stale dep short-circuits to :stale. All clean → update verified_at, return :valid.

Dependency inversion

Validation accepts an ensure_fn callback rather than depending on Runtime directly. Runtime provides its ensure_up_to_date/2 as the callback, breaking the compile-time dependency cycle (see D13).