feat(fhir): code a lab value without a unit as the UCUM unity

A value stored with unit "1" carries system and code on its quantity
even when the analyte map knows no coding for it: the unity is UCUM by
definition.

Refs #1313

src/lib/fhir/__tests__/lab-observations.test.ts

@@ -276,6 +276,16 @@ describe("lab-result LOINC + canonical UCUM coding (v1.18.8)", () => {
     expect(o?.valueQuantity?.unit).toBe("mg/dL");
   });
 
+  it("codes a value without a unit as the UCUM unity", () => {
+    const o = labOf("Cholesterol/HDL ratio", "1", 3.6);
+    expect(o?.valueQuantity).toEqual({
+      value: 3.6,
+      unit: "1",
+      system: "http://unitsofmeasure.org",
+      code: "1",
+    });
+  });
+
   it("resolves analyte aliases to the same canonical LOINC", () => {
     // "LDL", "LDL-C", "LDL Cholesterol" all fold to 2089-1.
     for (const name of ["LDL-C", "LDL Cholesterol", "ldl_c"]) {

src/lib/fhir/resources/labs.ts

@@ -21,6 +21,10 @@ import type { DoctorReportData } from "@/lib/doctor-report-data";
 import { LOINC_SYSTEM, UCUM_SYSTEM } from "@/lib/fhir/loinc-map";
 import { resolveLabCoding } from "@/lib/fhir/lab-loinc";
 import { qualitativeValueConcept } from "@/lib/fhir/lab-qualitative";
+import {
+  DIMENSIONLESS_UNIT,
+  isDimensionlessUnit,
+} from "@/lib/labs/dimensionless";
 
 import type {
   FhirObservation,
@@ -84,10 +88,15 @@ export function labObservations(
     // the code system while withholding the code claims a coding the bundle
     // does not carry, and leaves a receiver unable to machine-compare the
     // value it is attached to.
+    // A value without a unit is the UCUM unity by definition, so it carries
+    // the code even for an analyte the map does not know.
+    const ucum = isDimensionlessUnit(lab.unit)
+      ? DIMENSIONLESS_UNIT
+      : coding?.ucum;
     const quantity = (value: number) => ({
       value,
       unit: lab.unit,
-      ...(coding?.ucum ? { system: UCUM_SYSTEM, code: coding.ucum } : {}),
+      ...(ucum ? { system: UCUM_SYSTEM, code: ucum } : {}),
     });
     // The bounds are stated in the SAME unit as the value, so they carry the
     // SAME coding — a bare unit beside a coded value leaves a receiver unable