[dplyukhin]

What was changed

Added go=... annotations to nexus directives in WIT files.

Why?

To generate SIgnalWithStart.

Checklist

1. Closes: N/A
2. How was this tested:
   • make -B system-nexus-wit NEX_GEN=<local nexgen binary> passed and preserved all WIT files without regeneration changes.
   • Generated Python, TypeScript, and .NET output before and after the change and confirmed byte-for-byte equality for each language.
   • Generated the Go workflow bindings and compiled/tested them in a temporary SDK copy with the proposed internal support package.
   • The related nexgen changes passed full cargo validate.
   • Full API make ci-build was not run locally; validation here was WIT-specific.
3. Any docs updates needed?
   • The WIT annotations are the generated-API source. No separate documentation changes.