Add Apple container REPL and MLX backend

[bmorphism]

Summary

• add environment="apple" backed by Apple's container CLI
• add local backend="mlx" through optional mlx-vlm support
• reuse the Docker REPL host-proxy script with a configurable proxy host
• document the new Apple and MLX paths and cover them with focused tests

Validation

• uv run ruff check rlm/clients/__init__.py rlm/clients/mlx.py rlm/core/types.py rlm/environments/__init__.py rlm/environments/docker_repl.py rlm/environments/apple_container_repl.py tests/clients/test_mlx.py tests/repl/test_apple_container.py tests/test_types.py
• uv run ruff format --check rlm/clients/__init__.py rlm/clients/mlx.py rlm/core/types.py rlm/environments/__init__.py rlm/environments/docker_repl.py rlm/environments/apple_container_repl.py tests/clients/test_mlx.py tests/repl/test_apple_container.py tests/test_types.py
• uv run pytest tests/clients/test_mlx.py tests/repl/test_apple_container.py tests/test_types.py -q
• full suite: uv run pytest -q -> 284 passed, 8 skipped
• real Apple container smoke: RLM(..., environment="apple") on python:3.11-alpine returned 42
• recursive Apple container LM proxy: after configuring host.container.internal with Apple Container DNS, in-container llm_query("What is 2+2?") reached the host LMHandler and returned Mock: What is 2+2?; llm_query_batched(["Q1", "Q2"]) returned Mock: Q1|Mock: Q2

Scope

• Apple container REPL, MLX client, type/env plumbing, docs, tests, and lockfile only