fix(metadata): stop fabricating display metadata and add DICOM JSON converter

Replace the hand-built metadata subset in parseMeta with a VR-aware
DICOM-to-DICOM-JSON converter. Window Center/Width, Rescale Intercept/Slope
are only emitted when the source element exists; multi-values are preserved
in source order. VOI/LUT, Presentation LUT and Pixel Padding tags are
preserved and Pixel Data is excluded.

- add src/dimse/dicomJson.ts (dictionary-aware converter)
- rewrite src/dimse/parseMeta.ts to use the converter
- add synthetic non-PHI DICOM writer and metadata tests
- add npm test script (node:test + ts-node) and skipLibCheck

tests/dicomJson.test.ts

@@ -0,0 +1,114 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import dicomParser, { Element } from 'dicom-parser';
+import { convertElementToJson, DicomJsonDataSet, parseDicomToJson } from '../src/dimse/dicomJson';
+import { buildMonochromeImage, Dataset } from './helpers/dicomWriter';
+
+const BASE_PIXELS = [0, 1, 2, 3, 4, 5, 6, 7];
+
+function crLike(extra: Dataset = [], bufferExtra: Partial<Parameters<typeof buildMonochromeImage>[0]> = {}) {
+  return buildMonochromeImage({
+    rows: 2,
+    columns: 4,
+    pixelData: BASE_PIXELS,
+    extra,
+    ...bufferExtra,
+  });
+}
+
+test('omits Window Center/Width when the source tags are absent', () => {
+  const json = parseDicomToJson(crLike());
+  assert.equal(json['00281050'], undefined);
+  assert.equal(json['00281051'], undefined);
+});
+
+test('omits Rescale Intercept/Slope when the source tags are absent', () => {
+  const json = parseDicomToJson(crLike());
+  assert.equal(json['00281052'], undefined);
+  assert.equal(json['00281053'], undefined);
+});
+
+test('preserves multi-valued Window Center/Width in source order', () => {
+  const json = parseDicomToJson(crLike([
+    { tag: '00281050', vr: 'DS', value: '40\\400' },
+    { tag: '00281051', vr: 'DS', value: '80\\800' },
+  ]));
+
+  assert.deepEqual(json['00281050'].Value, [40, 400]);
+  assert.deepEqual(json['00281051'].Value, [80, 800]);
+});
+
+test('preserves rescale values when present', () => {
+  const json = parseDicomToJson(crLike([
+    { tag: '00281052', vr: 'DS', value: '-1024' },
+    { tag: '00281053', vr: 'DS', value: '2' },
+  ]));
+
+  assert.deepEqual(json['00281052'].Value, [-1024]);
+  assert.deepEqual(json['00281053'].Value, [2]);
+});
+
+test('preserves VOI, LUT, presentation LUT and pixel padding metadata', () => {
+  const json = parseDicomToJson(crLike([
+    { tag: '00281055', vr: 'LO', value: 'WINDOW' },
+    { tag: '00281056', vr: 'CS', value: 'LINEAR' },
+    { tag: '20500020', vr: 'CS', value: 'IDENTITY' },
+    { tag: '00280120', vr: 'US', value: 5 },
+    { tag: '00280121', vr: 'US', value: 10 },
+    {
+      tag: '00283010',
+      vr: 'SQ',
+      items: [
+        [
+          { tag: '00283002', vr: 'US', value: [256, 0, 8] },
+          { tag: '00283006', vr: 'US', value: [1, 2, 3] },
+        ],
+      ],
+    },
+  ]));
+
+  assert.deepEqual(json['00281055'].Value, ['WINDOW']);
+  assert.deepEqual(json['00281056'].Value, ['LINEAR']);
+  assert.deepEqual(json['20500020'].Value, ['IDENTITY']);
+  assert.deepEqual(json['00280120'].Value, [5]);
+  assert.deepEqual(json['00280121'].Value, [10]);
+
+  const sequence = json['00283010'];
+  assert.equal(sequence.vr, 'SQ');
+  const item = (sequence.Value as DicomJsonDataSet[])[0];
+  assert.deepEqual(item['00283002'].Value, [256, 0, 8]);
+  assert.deepEqual(item['00283006'].Value, [1, 2, 3]);
+});
+
+test('never emits Value: [undefined] for absent optional tags', () => {
+  const json = parseDicomToJson(crLike());
+  for (const [tag, element] of Object.entries(json)) {
+    if (Array.isArray(element.Value)) {
+      for (const value of element.Value) {
+        assert.notEqual(value, undefined, `${tag} contains an undefined value`);
+      }
+    }
+  }
+});
+
+test('excludes Pixel Data from metadata output', () => {
+  const json = parseDicomToJson(crLike());
+  assert.equal(json['7FE00010'], undefined);
+});
+
+test('falls back to the data dictionary when VR is not explicit', () => {
+  const buffer = crLike([
+    { tag: '00281050', vr: 'DS', value: '40' },
+  ]);
+  const dataSet = dicomParser.parseDicom(buffer);
+  const explicit = dataSet.elements.x00281050;
+  const implicitLike: Element = {
+    tag: explicit.tag,
+    length: explicit.length,
+    dataOffset: explicit.dataOffset,
+  };
+
+  const json = convertElementToJson(dataSet, implicitLike);
+  assert.equal(json.vr, 'DS');
+  assert.deepEqual(json.Value, [40]);
+});