fix: make build.rs example compile by adding anyhow imports and Resul…

[meelon-dev]

Summary

The build.rs example in proto/README.md was not compiling:

• ? was used in main() without main() returning a Result.
• .context(...) was used, but anyhow::Context was not imported.

Changes

• Added use anyhow::{Context, Result}; to imports.
• Changed fn main() to fn main() -> Result<()>.
• Added Ok(()) at the end of main() to satisfy the Result return type.

Reason

This makes the example compile correctly and demonstrates modern, idiomatic Rust error handling using anyhow.

Notes

• Only README/example code is affected.
• Works with users’ projects using anyhow and ? for concise error propagation.

[sergerad]

@Mirko-von-Leipzig does this readme need to spell out a build.rs contents at all?