Issue: Implement FixedSizeFlatMap with correct in-place initialization

Issue: Implement FixedSizeFlatMap with correct in-place initialization: a task in LegoFlow-SWE (Harbor dataset). We need a compile-time fixed-capacity associative container FixedSizeFlatMap<K, V, const CAPACITY: usize that is shared-memory compatible and uses the existing fixed-size slot-based…

The task

We need a compile-time fixed-capacity associative container `FixedSizeFlatMap<K, V, const CAPACITY: usize>` that is shared-memory compatible and uses the existing fixed-size slot-based storage. The container must behave like a map with unique keys, predictable failure modes, and proper drop semantics.

Part of Lego-X/LegoFlow-SWE.