refactor(venv): Extract pure VenvEngine from monolithic VenvManager

[raidos23]

Description

This PR refactors the virtual environment management system by extracting deterministic, pure Python logic from the monolithic VenvManager into a standalone, UI-independent VenvEngine.

Key Changes

1. New VenvEngine module (venvengine.py):

   • Isolated all pure Python, synchronous inspection and execution logic (no Qt/UI dependencies, no event loop expectations).
   • Handles path resolution, strict venv validation (validate_venv_strict), tool binary discovery (has_tool_binary), preference file I/O (read_workspace_pref/write_workspace_pref), dependency analysis (can_use_system_python), and SHA-256 requirement markers (compute_file_hash).
   • Added prepare_manager_command and create_venv_sync for CLI and headless execution.

2. Refactored VenvManager (Manager.py):

   • VenvManager now acts as an orchestrator / facade, delegating pure logic to self.engine while preserving 100% backwards compatibility for existing GUI/CLI flows.
   • Bug Fix: Fixed a double invocation loop in install_requirements_if_needed that previously triggered create_venv_if_needed twice during pycompiler_ark init --with-venv.

3. Unit Tests (tests/test_venv_engine.py):

   • Added a dedicated test suite for VenvEngine testing path resolution, strict validation, requirement markers, command preparation, and preference management.

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Verification and Testing

• All 34 unit tests passed cleanly (python3 -m unittest discover -s tests).
• Verified pycompiler_ark init --entry ark.yml --with-venv CLI execution without duplicate log output.

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Checklist

• Code is properly typed and formatted.
• No breaking changes to the existing VenvManager public API.
• Unit tests added and passing.