feat: support pre-risk strategy hook

[stefan-jansen]

Summary

• add Strategy.on_before_risk() so strategies can submit opening orders before position-risk evaluation
• allow position rules to be explicitly cleared globally or per asset
• add trail_include_entry_bar_extremes to opt into using entry-bar extremes for next-bar trailing watermarks
• preserve risk-exit metadata through order creation options

Verification

• pre-commit run --all-files
• uv run ty check
• uv run pytest tests/ -q
• uv run mkdocs build --strict
• uv build

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