Function _buildCheckFormula

_buildCheckFormula(
    baseFormula: string,
    choice: "roll" | "take10" | "decline",
    advantageStage: number,
): string
Apply advantage/disadvantage stage and take-10 substitution to a roll formula.

Rules:
- Success-dice formulas (contain cs>=) are never modified — pass advantageStage = 0 for those.
- take10 replaces the leading d20 with 10 regardless of stage.
- stage > 0: d20 → Nd20kh1 (keep highest, N = 1 + stage).
- stage < 0: d20 → Nd20kl1 (keep lowest, N = 1 + |stage|).
- stage 0, not take10: formula unchanged.
Parameters
- baseFormula: string
- choice: "roll" | "take10" | "decline"
- advantageStage: number
  Pass 0 for success-dice formulas.
Returns string
- Defined in module/apps/compel-check.mjs:468