[M4] Python custom engine (trampoline)

[tetsuh]

Summary

Support custom storage engines written in Python via a nanobind trampoline.

References

• docs/05_api_python.md §2.3 (custom engine contract)
• docs/01_requirements.md X01, N03

Scope

• python/bindings/storage_engine_trampoline.cpp: subclass
  sitos.StorageEngine in Python implementing
  put/get/list/delete (and optional take_snapshot)
• Missing take_snapshot falls back to the copy-based default (N03)
• GIL handling documented: node calls into Python engines take the GIL;
  production guidance (prefer C++ engines) in the docstring
• pytest version of the engine contract suite, instantiated with a
  Python dict-based engine (tests/python/test_custom_engine.py)

Out of scope

• Performance parity with C++ engines

Acceptance Criteria

1. A pure-Python engine passes the pytest contract suite
2. Snapshot isolation holds via the fallback for engines without
   take_snapshot
3. A StorageNode running on a Python engine serves gets/puts end-to-end

Dependencies

• [M4] Python StorageNode / SessionView #25

For the implementer

• Copy the checklist above into your PR description and tick items as you
  complete them. Do not edit the checklist in this issue.
• Follow docs/development_workflow.md (branch issue/28-python-custom-engine,
  Conventional Commits per §2.1, TDD RED first).
• This issue is closed automatically by the PR via Closes #28.