Fix saved simulation reloads and live-viewer cancellation

[georgeoshardo]

What changed

• Let the custom Pymunk joint classes initialize from either SyMBac cell configuration or the base constructor arguments stored by Pymunk. This restores real saved spaces containing pivot, rotary-limit, and damped-spring joints.
• Treat closing the live viewer while its worker is active as cancellation. Shutdown now requests a stop, joins the worker without a timeout, and returns without replacing the canonical simulation pickles.
• Add a one-frame real Simulation save/reload regression and a real-thread cancellation lifecycle regression without opening a GUI.

Root causes

Pymunk unpickles constraints by calling the concrete subclass with its serialized base constructor arguments. The SyMBac wrappers only accepted (segment_a, segment_b, config), so loading a real space_timeseries.p raised TypeError.

The live-viewer cleanup waited at most one second for the worker, then always wrote canonical artifacts. A slow in-progress frame could therefore keep mutating simulation state while a partial run was saved as complete.

Validation

• pixi run pytest -q tests/test_simulation_persistence.py tests/test_live_viewer.py tests/test_simcell_core.py tests/test_division_manager_split.py — 19 passed
• pixi run pytest -q — 65 passed, with the existing CPU FFT fallback warning from SyMBac/renderer.py