Add RetrieveCb method to ConcurrentMap to allow inspecting or deep copying values (especially pointer and reference types) safely while holding the shard's read lock (RLock).

Motivation

When storing pointer values in ConcurrentMap, retrieving a value via Get() returns the raw pointer. If another goroutine concurrently modifies the contents of that pointer or performs in-place mutation, data races can occur. RetrieveCb provides a callback executed while holding RLock on the shard, matching the pattern of Upsert and RemoveCb:

m.RetrieveCb("key", func(key string, val *MyStruct, exists bool) {
    if exists {
        // safely read or clone under RLock
    }
})

• Added RetrieveCb[K any, V any] type and method on ConcurrentMap.
• Added unit tests in concurrent_map_test.go.
• All tests pass 100%.

Fixes #161