Support affine unit conversions in unit_conversions.json

[BorisDelange]

units/unit_conversions.json currently stores only multiplicative conversion factors (target = factor × source). This works for most analytes but cannot represent affine conversions of the form target = factor × source + offset.

Known cases that need affine conversions

• HbA1c: IFCC [mmol/mol] = (NGSP [%] − 2.152) / 0.0915 (derived from the NGSP-IFCC master equation, see concept set 184).
• Temperature: Fahrenheit ↔ Celsius (°F = 9/5 × °C + 32).

For HbA1c, the conversion is currently documented in the concept set description and must be applied manually in downstream code.

Proposed change

Add an optional offset field to each conversion entry. When absent or 0, the conversion remains purely multiplicative — so the 372 existing entries are unaffected.

{
  "conceptId": 3004410,
  "sourceUnitConceptId": 8554,
  "sourceUnitCode": "%",
  "conversionFactor": 10.929,
  "offset": -23.52,
  "targetUnitConceptId": 9579,
  "targetUnitCode": "mmol/mol",
  ...
}

target = factor × source + offset