## `concurrent_tee` is missing from `more_itertools`

### Description

There is no `concurrent_tee` function available in `more_itertools`, even though it is expected to exist as a thread-safe variant of `itertools.tee`. When trying to use it, you get an `AttributeError`.

Here's a minimal example that reproduces the issue:

```python
import more_itertools as mi
from threading import Thread, Lock

def producer(limit):
    for x in range(limit):
        yield x

limit = 10**5
num_threads = 100
non_concurrent_source = producer(limit)
tees = mi.concurrent_tee(non_concurrent_source, n=num_threads)
```

This raises:

```
AttributeError: module 'more_itertools' has no attribute 'concurrent_tee'
```

### Expected behavior

`concurrent_tee` should take a non-thread-safe iterable and return `n` independent iterator copies that can each be safely consumed in separate threads. All returned iterators should share a single lock so that concurrent calls to `__next__` on different tee objects are properly serialized against the underlying source.

Additionally:
- Passing `n=0` should return an empty tuple `()`.
- Passing a negative `n` should raise a `ValueError`.

The function should guarantee that each consumer thread receives the complete, correct sequence of values from the source (no duplicates, no dropped items), even when 100 threads consume their copies concurrently.

### Actual behavior

Calling `mi.concurrent_tee(...)` raises `AttributeError: module 'more_itertools' has no attribute 'concurrent_tee'` because the function has not been implemented and exported from the module.

Work in `/workspace`. Submit your fix in the existing Python source files under `more_itertools`. Preserve the other public behavior. The environment is offline; dependencies are preinstalled. Grading runs the repository's test suite in a fresh environment, using your submitted source files.