Add risk calculators

[simonwhatley]

• Added shared LLPv2 and PLCOm2012 calculator modules under app/lib/risk-calculators.
• Implemented calculator calculate() functions without wiring them into prototype journeys.
• Added shared input validation, standard result formatting, and calculator error types.
• Added JSDoc for the calculator APIs, inputs, errors and helper functions.
• Documented the implementation in docs/risk-calculators.md using NHS style guidance.

Important notes

• Calculator outputs are for prototype use only.
• They are not clinically signed off and must not be used for clinical decisions.
• Eligibility thresholds are not applied inside the calculators. Prototype-specific adapter or journey code should own threshold rules and routing.