build(fs_connector): ship cp38-abi3 stable-ABI wheels (currently locked to cp312)

[kfirtoledo]

Problem

fs_backend wheels are tagged cp312-cp312 — locked to one Python
version. vLLM publishes cp38-abi3 wheels, where one .so works on any
cpython 3.8+.

PR #566 tried this and hit a wall: torch 2.10's bundled pybind11 uses
non-stable-API macros (PyTuple_SET_ITEM), so Py_LIMITED_API breaks
every <torch/extension.h> consumer. We ship cp312 today and defer
abi3 to this issue.

Path forward

Bindings need to stop pulling torch's pybind11. Two options:

• TorchBind: swap pybind11::class_<> for torch::class_<>
  (dispatcher path). Smaller diff; setup.py stays.
• CMake + system pybind11 (vLLM's approach): bigger build-system
  rewrite, more flexibility.

Acceptance

• Wheel: llmd_fs_connector-...-cp38-abi3-manylinux_2_35_<arch>.whl
• Installs/imports on cpython 3.10–3.13
• Existing storage offload tests pass