Overview

Case enums (Swift enums without raw values or associated values) could not be used as parameters or return values of imported (@JSFunction) signatures. They already bridge across the export boundary as their Int32 tag, but the import path rejected them:

@JS enum Signal { case start; case stop }

@JSFunction func send(_ signal: Signal) throws(JSException)      // "Enum types are not yet supported in TypeScript imports"
@JSFunction func current() throws(JSException) -> Signal         // same error

The JavaScript glue already round-trips the Int32 tag in both directions; only the import-side type lowering and lifting in the code generator threw instead of producing the tag transfer.

This change enables case enums as imported parameters and return values by lowering and lifting their Int32 tag in the import context, identical to how the export side already handles them. The bridged representation is the case index, with the JS side exposing the value table and a numeric tag type in the generated .d.ts.

Adds an end-to-end runtime round-trip test and a codegen snapshot covering the new import shape.