feat(base-formula): add function registry register helper

packages/base-formula/src/functions/registry.ts

@@ -12,3 +12,10 @@ export type FormulaFn = {
 };
 
 export const registry: Map<string, FormulaFn> = new Map();
+
+export function register(fn: FormulaFn): void {
+  if (registry.has(fn.name)) {
+    throw new Error(`Duplicate formula function: ${fn.name}`);
+  }
+  registry.set(fn.name, fn);
+}

packages/base-formula/src/index.server.ts

@@ -7,6 +7,6 @@ export * from "./parser";
 export * from "./resolver";
 export * from "./typecheck";
 export * from "./format";
-export { registry } from "./functions/registry";
+export { registry, register } from "./functions/registry";
 export type { FormulaFn } from "./functions/registry";
 export * from "./graph";