Fix: simulation result does not get saved

[Dhiyaahaq33]

Summary

• SimulationCalculator._save_calculation_results wrote 4 of the 5 state pickles (asset_record, unrealized_changes, scribbles, account_state) but never virtual_state.pickle, despite a comment claiming this was intentional.
• _load_or_create_previous_state unconditionally tries to read all 5 pickle files inside a single try block. Since virtual_state.pickle never existed, every load raised FileNotFoundError, which was caught and treated as "no previous state" — discarding the other 4 correctly-saved pieces of state too and resetting calculate_from back to the start of the year.
• Net effect: simulation progress never actually persisted across runs, matching the report in Simulation result does not get saved #146.
• Fix: virtual_state is now threaded through CalculationResult / _merge_calculation_results and written to disk in _save_calculation_results, restoring parity with _load_or_create_previous_state's expectations (and with the pre-refactor implementation, which did save it).

Fixes #146

Test plan

• ruff check package/solie/logic/simulation_calculator.py passes
• python -m py_compile on the changed file passes
• Manual verification in the running app: run a simulation, close and reopen the year/strategy tab (or restart the app), confirm the calculation resumes from where it left off instead of recalculating from the start of the year — happy to do this if a maintainer can point me at the quickest way to set up a data folder for testing.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WJ386AEgV8Z796N4TSJtPn