[codex] Document live risk budget stance

[Pigbibi]

Summary

• document that soxl_soxx_trend_income remains live as the high-return semiconductor engine
• clarify that us_equity_combo_leveraged_shadow_402040 is a replacement candidate for part of existing TQQQ/SOXL exposure, not an additive live layer
• keep TECL, CN combo, and HK combo in research/shadow pending separate evidence and approval

Validation

• git diff --check → passed
• doc content smoke check for SOXL and 402040 references → passed

Deployment

• Documentation-only change. No runtime config or Cloud Run deployment required.

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