feat(executor): restore pool_executors package and remove upstream shim

1. **Unify executor creation**: Single factory function for all executor types with consistent interface

1. **Enable future extraction**: Designed as standalone package for easy repository separation

## Subinterpreter Support

### Why Subinterpreters?

Python's Global Interpreter Lock (GIL) and thread-safety issues in many C extensions limit the effectiveness of standard threading. Subinterpreters provide an alternative approach:

- **Thread Safety**: Each subinterpreter has its own GIL, providing true isolation between tasks

- **Performance**: Similar to real threading without the thread-safety concerns of shared-memory threading

- **Encapsulation**: Each subinterpreter runs in complete isolation with its own interpreter state

### Subinterpreters vs Other Execution Strategies

| Strategy | Pros | Cons |

|----------|------|------|

| **Serial** | No overhead, deterministic, easy debugging | No parallelism |

| **Threading** | Low overhead, shared memory | GIL contention, thread-safety issues in many libraries |

| **Multiprocessing** | True parallelism, no GIL | High inter-process communication overhead, especially on Windows |

| **Subinterpreter** | Thread-safety isolation, good performance | Python 3.14+ required, still maturing |

### Future Outlook

As Python 3.14+ matures and more libraries become subinterpreter-safe, this approach will provide a clean path toward concurrent execution without the overhead of multiprocessing. Until then, using well-encapsulated subinterpreters offers a good balance between thread-safety and performance characteristics similar to real multi-threading.

### Fallback Behavior

When `InterpreterPoolExecutor` is not available (Python < 3.14), the factory automatically falls back to `ProcessPoolExecutor` with a warning log message.

## Requirements

- Python 3.13 or higher

- Python 3.14+ recommended for subinterpreter support

- No external dependencies (stdlib only)

## License

"""Executor pool extensions with serial execution support."""

from pool_executors.factory import create_executor

from pool_executors.serial import SerialPoolExecutor

from pool_executors.types import ExecutorType

from .factory import create_executor

from .serial import SerialPoolExecutor

from .types import ExecutorType

__version__ = "0.1.0"

from __future__ import annotations

import logging

from concurrent.futures import (Executor, ProcessPoolExecutor,

                                ThreadPoolExecutor)

from concurrent.futures import Executor, ProcessPoolExecutor, ThreadPoolExecutor

from typing import Any

from pool_executors.serial import SerialPoolExecutor

from pool_executors.types import ExecutorType

from .serial import SerialPoolExecutor

from .types import ExecutorType

# Import InterpreterPoolExecutor for Python 3.14+, handle gracefully for older versions

try:

    from concurrent.futures import \

        InterpreterPoolExecutor  # type: ignore[attr-defined]  # noqa: PLC0415

    from concurrent.futures import InterpreterPoolExecutor  # noqa: PLC0415

    HAS_INTERPRETER_POOL_EXECUTOR = True

except (ImportError, AttributeError):

from __future__ import annotations

import contextlib

import logging

from collections.abc import Callable

from concurrent.futures import Executor, Future

from concurrent.futures import Executor

from typing import Any, TypeVar

from tdoc_crawler.pool_executors import create_executor

from pool_executors.pool_executors import create_executor

T = TypeVar("T")