write vital-sign Observations to FHIR R4 with correct LOINC codes and UCUM units

Steps

1. Set Observation.status to 'final' (or 'amended' for corrections) and Observation.category to a coding with system http://terminology.hl7.org/CodeSystem/observation-category and code 'vital-signs'.
2. Populate Observation.code with the appropriate LOINC code for the vital sign (e.g. 8867-4 for heart rate, 8310-5 for body temperature, 55284-4 for blood pressure panel) and system http://loinc.org.
3. For simple numeric values, set valueQuantity.value, valueQuantity.unit (display string), and valueQuantity.system to http://unitsofmeasure.org with valueQuantity.code set to the canonical UCUM code (e.g. /min, Cel, mm[Hg]).
4. For blood pressure (a panel), use Observation.component[] with separate entries for systolic (8480-6) and diastolic (8462-4), each with their own valueQuantity.
5. Set Observation.subject to a reference to the Patient resource and Observation.effectiveDateTime to the ISO 8601 timestamp of the measurement.
6. POST to {base}/Observation or include in a transaction Bundle; validate the resource against the US Core Vital Signs profile before submission.