Prototype multi threaded file reader (#343)

- Reading files from nfs shares tops out at ~1GB/s for single threaded
reads.
- MultiThreadedFileReader uses our File wrapper to read a generic file
in parallel
- Placed in aare::experimental to show that it's not production ready

docs/src/python/experimental/MultiThreadedFileReader.rst

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+MultiThreadedFileReader
+=======================
+
+.. py:currentmodule:: aare.experimental
+
+The reader returns a NumPy array with shape ``(frames, rows, cols)`` and
+preserves the source pixel dtype. Each iteration reads at most
+``n_threads * chunk_size`` frames—one chunk per worker. File I/O runs with the
+Python GIL released.
+
+.. code-block:: python
+
+    from aare.experimental import MultiThreadedFileReader
+
+    with MultiThreadedFileReader(
+        "frames.npy", n_threads=4, chunk_size=128, total_frames=10_000
+    ) as reader:
+        for frames in reader:
+            process(frames)
+
+Call ``read()`` directly for the next batch, or ``read_all()`` for all frames
+remaining from the current position. ``tell()`` and ``seek()`` expose the
+iteration position. The context manager closes all worker files on exit.
+``close()`` is also available for explicit cleanup and may be called
+repeatedly.
+
+.. autoclass:: MultiThreadedFileReader
+    :members:
+    :undoc-members:
+    :show-inheritance:

docs/src/python/experimental/index.rst

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+Experimental
+============
+
+APIs in this module may change without notice.
+
+.. toctree::
+   :caption: Experimental
+   :maxdepth: 1
+
+   MultiThreadedFileReader