Persist last used variables in the store

[wtmoose]

From the Query.get() function:

// WORKAROUND: Special case for VariableQueryRepository and ConstantQueryRepository
// When QueryId == Variables (detected by successful casting and equality), the variables
// are effectively constant for a given queryId, so variablesChanged should always be false.
// This fixes the issue where queries are unnecessarily repeated after app restarts because
// the in-memory `lastVariables` state is lost while the persisted store data remains.
//
// IDEAL SOLUTION: Enhance TimestampedValue to store variables alongside the value, and
// add a `lastVariables(of:)` method to the Store protocol. This would ensure variables
// lifetime exactly matches the stored value lifetime, eliminating this entire class of issues.
// The ideal approach would involve:
// 1. TimestampedValue<Value, Variables> - store variables with each cached value
// 2. Store.lastVariables(of:) -> Variables? - retrieve variables for any stored key  
// 3. Enhanced all store implementations (FileStore, DictionaryStore, etc.)
// This would be architecturally perfect but requires significant breaking changes.
if let queryIdAsVariables = id as? Variables, queryIdAsVariables == variables {
    variablesChanged = false
} else {
    let latestVariables = await latestVariables(for: id)
    variablesChanged = variables != latestVariables
}