feat: model deterministic trade settlement

[fallblu]

Summary

• separate trade-date economic accounting from settlement-date cash and position availability
• add versioned settlement calendars, per-instrument lags, deterministic instructions, failures, and explicit buying-power limits
• publish scenario/journal contract v11 and strategy protocol v9 with frozen v10/v8 compatibility
• add golden fixtures, conformance coverage, documentation, and end-to-end settlement tests

Relates to #30.

Test plan

• make check
• make coverage (86.01%)
• schema and strategy transcript validation for v11/v9
• deterministic journal verification for v3 through v11