Extend Strategy Spec feature selectors for signal features

[sybapp]

What to build

Extend Strategy Spec feature selectors so strategies can explicitly consume derived Candidate Signal features without accidentally matching raw TradingView graphics or plots.

Acceptance criteria

• Strategy Spec feature selectors support an optional type field alongside indicator id and feature name.
• Existing Strategy Specs without type continue to validate and replay unchanged.
• LuxAlgo ICT/SMC Strategy Specs can require type: "signal" and value: true for entry rules.
• Evaluator Smoke Replay and Nautilus Validation both honor the selector type when present.
• Tests cover backward compatibility and a signal-feature-only entry rule.

Blocked by

• Derive directional Structure Event signals from LuxAlgo graphics #23