Summary:

Adds the backend-agnostic machinery for moving large binary payloads (large `bytes`, NumPy arrays, Torch tensors) between the worker and parent of `iterate_in_subprocess` through shared memory instead of pickling them through the result queue. This commit contains only the framework; concrete buffer backends are added in following commits.

New private subpackage `spdl/pipeline/_arena/`:

- `_protocol.py`: structural `ArenaProtocol` / `ArenaWriterProtocol` / `ArenaReaderProtocol` interfaces. `iterate_in_subprocess` accepts any object satisfying `ArenaProtocol` through one `buffer` argument and dispatches on its type, so additional backends need no signature change. A unit (one pipeline item, possibly several binaries) is bracketed by `begin_unit` / `commit_unit` on the writer and returned with `end_unit` on the reader, so reservation and return happen in bulk per unit.
- `_marker.py`: `_ShmMarker`, the small descriptor that replaces an offloaded binary in the pickled envelope.
- `_registry.py`: decides which objects are offloaded and how. A handler exposes an object's bytes as a zero-copy source buffer (`get_buffer`) so the arena copies into shared memory exactly once, and rebuilds it from a destination buffer (`from_buffer`), returning the restored object plus a lifetime anchor for zero-copy views. The `bytes` handler is pure Python; NumPy and Torch are reached through `lazy_import` so `import spdl.pipeline` works without them and they never become hard dependencies.
- `_offload.py`: `offload` / `restore`, which divert offloadable leaves of an arbitrarily nested object into the arena via a pickler `persistent_id` / `persistent_load` pair while the rest is pickled normally. The envelope carries an 8-byte unit span so the reader can return the whole region in one call.

`iterate_in_subprocess` gains a `buffer: ArenaProtocol | None` argument. When provided, the worker offloads each item before putting it on the queue and the parent restores it as the first step; the arena is created and owned by the parent, passed to the worker, and closed and unlinked at teardown after the worker is confirmed dead. With `buffer=None` the behavior is unchanged.

Reviewed By: huangruizhe

Differential Revision: D107709116