chore(base-foundation): replace Memoizer with a typed hierarchy and builder

[deer]

• Deletes the single build.base.foundation.Memoizer class and replaces it with a new build.base.foundation.memoizer package containing a Memoizer<T,R> interface, a fluent builder, and six concrete implementations covering every combination of reference mode (strong / soft / weak) and threading model (concurrent / non-concurrent).
• The builder API is Memoizer.of(fn).soft().concurrent().build() — reference mode and concurrency are opt-in, making intent explicit at each call site.
• AbstractConcurrentMemoizer uses a per-key ReentrantLock rather than ConcurrentHashMap.computeIfAbsent for the value cache, which allows memoized functions to safely re-enter the memoizer for keys that share a hash bin without deadlocking.
• All call sites in Introspection and AbstractHeapBasedIndex are updated to the new API; tests are expanded to a parameterized suite covering all six implementations, including concurrent-load and re-entrancy scenarios.