feat(converters): add reverse lookup for supported source formats

src/converters/main.ts

@@ -282,6 +282,37 @@ export const getPossibleTargets = (from: string): Record<string, string[]> => {
   return possibleTargets[fromClean] || {};
 };
 
+// Reverse of possibleTargets: for a given output extension, which input
+// extensions (across all converters) can produce it, grouped by converter.
+const possibleSources: Record<string, Record<string, string[]>> = {};
+ 
+for (const converterName in properties) {
+  const converterProperties = properties[converterName]?.properties;
+  if (!converterProperties) continue;
+ 
+  for (const key in converterProperties.from) {
+    const fromList = converterProperties.from[key];
+    const toList = converterProperties.to[key];
+ 
+    if (!fromList || !toList) continue;
+ 
+    for (const target of toList) {
+      if (!possibleSources[target]) possibleSources[target] = {};
+ 
+      const existing = possibleSources[target][converterName];
+      possibleSources[target][converterName] = existing
+        ? Array.from(new Set([...existing, ...fromList]))
+        : [...fromList];
+    }
+  }
+}
+ 
+export const getPossibleSources = (to: string): Record<string, string[]> => {
+  const toClean = normalizeOutputFiletype(to);
+ 
+  return possibleSources[toClean] || {};
+};
+
 const possibleInputs: string[] = [];
 for (const converterName in properties) {
   const converterProperties = properties[converterName]?.properties;