[FEATURE]: Python bindings (PyO3)

[terylt]

Epic: CPEX Rust Core (#12)

Summary

Create cpex-python crate providing PyO3 bindings so Python code can call the Rust PluginManager. This is the "Python → Rust" direction — Python hosts (like ContextForge gateway) use the Rust core for execution.

Motivation

The ContextForge gateway is written in Python. PyO3 bindings let it use the Rust PluginManager as a drop-in replacement, getting compile-time invariant enforcement and the typed hook system while keeping the existing Python integration code.

Scope

• #[pymodule] exposing PyPluginManager
• PyPluginManager — wraps cpex_core::PluginManager with Python-callable methods
• invoke_hook(hook_name, payload, context, extensions) — Python calls, Rust executes
• PyO3-wrapped Rust types for payloads and extensions — Python accesses attributes, Rust controls access
• Maturin build configuration (pyproject.toml → cpex._native module)
• Type stubs (_native.pyi) for IDE support
• cpex/__init__.py import switch — Rust backend if available, Python fallback otherwise

Acceptance Criteria

• from cpex import PluginManager loads the Rust-backed manager when compiled
• manager.invoke_hook() dispatches through the Rust 5-phase executor
• Python plugins receive PyO3-wrapped payloads with typed attributes
• Existing PluginConfig YAML format works without changes
• Falls back to pure Python implementation if Rust module not compiled