feat(base-formula): scaffold shared package

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

@@ -0,0 +1,3 @@
+// Client-side public surface: parse, typecheck, cycle-detect, pretty-print.
+// Does NOT export eval or the function registry.
+export {};

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

@@ -0,0 +1,2 @@
+// Server-side public surface: everything in client + evaluator + registry.
+export {};