This PR supercedes #1334.

It introduces SafeMetadata, an immutable, thread-safe wrapper around io.grpc.Metadata, solving the ergonomics issues blocking the original PR.

Key Changes:

• Added kyo.grpc.SafeMetadata (Immutable Algebra).
• Refactored RequestOptions, ClientCall, and ServerCallHandlers to use potentially-unsafe io.grpc.Metadata ONLY at the IO boundary.
• Updated benchmarks to match new API.

Closes #390 /claim #390