Fix doc comment space in generated field docs

[kostakost2]

Fixes #4.

Problem

prettyplease renders #[doc = "X"] verbatim as ///X — no space after the slashes. This produces generated output like:

///Field 1: `sentence`
pub sentence: &'a str,

Fix

Prepend a single space to the three field doc format strings in the codegen so the emitted #[doc = " Field ..."] renders as /// Field ... per rustdoc convention.

// before
let tag_line = format!("Field {field_number}: `{field_name}`");

// after  
let tag_line = format!(" Field {field_number}: `{field_name}`");

Three sites changed: message.rs:1062, view.rs:318, view.rs:329.

cargo build --package buffa-codegen passes. Full test suite requires protoc which is unavailable in my environment, but the changes are mechanical and self-contained.

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