fix(backtest): create artifacts dir before writing validation.json

[isaveall]

What

When config["validation"] is set, BaseEngine.run_backtest() writes run_dir/artifacts/validation.json (step 7) before _write_artifacts() creates run_dir/artifacts (step 8). If that directory doesn't already exist, the write raises FileNotFoundError and aborts the backtest.

This surfaced when running an investment_committee swarm — the portfolio_manager sub-agent's run_dir has no pre-created artifacts/, so its validation-enabled backtest crashed (the swarm was fault-tolerant and still finished, but the PM's backtest artifact was lost).

Fix

Ensure the parent directory exists immediately before the write:

v_path = run_dir / "artifacts" / "validation.json"
v_path.parent.mkdir(parents=True, exist_ok=True)
v_path.write_text(...)

One line + a short explanatory comment. The bug is not platform-specific.

Test

Adds TestValidationArtifactDir to tests/test_engine_robustness.py: runs a backtest with validation enabled against a run_dir that has no pre-created artifacts/, and asserts validation.json is written.

• With the fix: passes.
• Reverting only the fix: fails with the exact FileNotFoundError.
• Full tests/test_engine_robustness.py: 32 passed.

Fixes #427

[warren618]

Belated note for the record: merged as-is (7311d6b) — clean 4-line fix, verified locally with a regression test before landing. Thanks!