feat: add source-only dfont font converter

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/converters/dfont.js

@@ -0,0 +1,52 @@
+import fs from 'node:fs/promises';
+import * as fontkit from 'fontkit';
+import { Font } from 'fonteditor-core';
+import { register } from './registry.js';
+import { convertBufferToOtf } from './font.js';
+
+export function probeDfont(buffer) {
+  try {
+    const dfont = fontkit.create(buffer);
+    const types = dfont.header?.map?.typeList?.types ?? [];
+    return types.some((t) => t.name === 'sfnt');
+  } catch {
+    return false;
+  }
+}
+
+export function extractSfntBuffer(buffer) {
+  const dfont = fontkit.create(buffer);
+  const sfntType = dfont.header.map.typeList.types.find((t) => t.name === 'sfnt');
+  const ref = sfntType.refList[0];
+  const lenPos = dfont.header.dataOffset + ref.dataOffset;
+  const len = buffer.readUInt32BE(lenPos);
+  const start = lenPos + 4;
+  return buffer.subarray(start, start + len);
+}
+
+function sniffSfntType(buffer) {
+  return buffer.slice(0, 4).toString('ascii') === 'OTTO' ? 'otf' : 'ttf';
+}
+
+export function registerDfontConverter() {
+  for (const targetFormat of ['ttf', 'otf', 'woff']) {
+    register({
+      family: 'font',
+      sourceFormat: 'dfont',
+      targetFormat,
+      convert: async (inputPath, outputPath) => {
+        const dfontBuffer = await fs.readFile(inputPath);
+        const sfntBuffer = extractSfntBuffer(dfontBuffer);
+
+        if (targetFormat === 'otf') {
+          await convertBufferToOtf(sfntBuffer, outputPath);
+        } else {
+          const sourceFormat = sniffSfntType(sfntBuffer);
+          const font = Font.create(sfntBuffer, { type: sourceFormat });
+          const out = font.write({ type: targetFormat });
+          await fs.writeFile(outputPath, Buffer.from(out));
+        }
+      },
+    });
+  }
+}