mason: preserve OpenCode text-before-tool order

Match the Rust native projection to OpenCode's structural-part semantics and use exact positional identity only when the complete unstamped adapter vector agrees. This updates tagged text in its original native part instead of appending it after a tool call.\n\nThe native-envelope preservation expectation now keeps the original Anthropic metadata and timing because the parity alignment intentionally retains those bytes.

Co-Authored-By: Alfonso <alfonso@cortexkit.io>