feat: add converter registry

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

src/converters/registry.js

@@ -0,0 +1,24 @@
+let entries = new Map();
+
+function key(sourceFormat, targetFormat) {
+  return `${sourceFormat}->${targetFormat}`;
+}
+
+export function register({ family, sourceFormat, targetFormat, convert }) {
+  entries.set(key(sourceFormat, targetFormat), { family, convert });
+}
+
+export function resolve(sourceFormat, targetFormat) {
+  return entries.get(key(sourceFormat, targetFormat)) ?? null;
+}
+
+export function listTargetFormats(sourceFormat) {
+  const prefix = `${sourceFormat}->`;
+  return [...entries.keys()]
+    .filter((k) => k.startsWith(prefix))
+    .map((k) => k.slice(prefix.length));
+}
+
+export function _resetForTests() {
+  entries = new Map();
+}