chore(deps): drop unused MLX dependency

MLX was declared as a darwin-only dependency but no source code
imports it — runtime is NumPy (Python path) + Rust/Rayon
(CORE_BACKEND=rust). Per ADR-0020, third-backend work
(GPU / JAX / MLX) is explicitly deferred until both Python and
Rust paths are mature.

Smoke suite (27/27) confirms no path depends on MLX. Re-add if
and when a deferred Apple-Silicon GPU backend is opened.

pyproject.toml

@@ -6,7 +6,6 @@ requires-python = ">=3.11"
 
 dependencies = [
     "numpy>=1.26",
-    "mlx>=0.18; sys_platform == 'darwin'",
     "pytest>=9.0.3",
     "pytest-asyncio>=1.3.0",
     "ruff>=0.15.12",