Extract retry and gate helpers from stage_tournament

[jayscambler]

Summary

stage_tournament() in mts/src/mts/loop/stages.py mixes evaluation, retry policy, gate selection, prompt rewriting, persistence, and event emission in one control-heavy function. The current structure is harder to test, harder to optimize, and harder to modify safely.

Scope

1. Split stage_tournament() into focused components/helpers such as:
   • tournament evaluator/runner
   • retry strategy
   • gate decision resolver
   • recovery/event recorder
2. Reduce broad except Exception blocks by isolating failure boundaries
3. Preserve current semantics for rapid mode, trend-aware gate, retry learning, and replay generation
4. Add targeted tests around the extracted pieces instead of only end-to-end stage behavior

Acceptance Criteria

• stage_tournament() becomes materially smaller and mostly orchestration-only
• retry logic, gate resolution, and persistence/event side effects are separated into testable units
• rapid mode and standard mode behavior remain unchanged
• regression tests cover retry and gate behavior after refactor

Files

• mts/src/mts/loop/stages.py
• related generation pipeline tests under mts/tests/

[linear]

MTS-145