test: add unit tests for variable persistence (Variables.Load/Save)

Add persistence_test.go with 12 tests covering:

- Load from corrupt JSON file returns proper error
- Load filters invalid variable names (empty, spaces, symbols)
- Round-trip Save/Load with 9 diverse float64 values (zero, negative,
  large, small, scientific, pi, integer, decimal)
- Save produces sorted, indented JSON format
- Save/Load with 50 variables
- Concurrent SetVariable/GetVariable/Count/ListVariables on same instance
- Load from empty JSON array succeeds with empty result
- Load from corrupt file preserves existing variables (error before lock)
- Save with Inf values returns error (JSON cannot represent Inf)
- Load with duplicate names — last value wins
- Load from empty file returns error (invalid JSON)

0 files changed, 0 insertions, 0 deletions