Downstream generators have to borrow GeneratedFileKind::Owned for non-message content

[iainmcgin]

Downstream code generators that emit per-proto content alongside buffa's output (service stubs, extra trait impls, etc.) currently have to tag their files as GeneratedFileKind::Owned so the per-package stitcher merges them into <stem>.rs. That works only because the stitcher matches by filename, and it's semantically misleading - the content isn't owned-message definitions.

connect-rust does this today (connectrpc-codegen/src/codegen.rs):

out.push(GeneratedFile {
    name: format!("{}.rs", buffa_codegen::proto_path_to_stem(file_name)),
    package: file.package.clone().unwrap_or_default(),
    kind: GeneratedFileKind::Owned,   // borrowed tag - not actually Owned content
    content: service_code,
});

A dedicated variant (or an explicit "append to this file" hint) would let downstream generators participate without overloading Owned:

pub enum GeneratedFileKind {
    Owned, View, Oneof, ViewOneof, Ext, PackageMod,
    /// Extra per-proto content from a downstream generator. The stitcher
    /// `include!`s it after the Owned file for the same stem.
    Extension,
}

or alternatively a merge_into: Option<String> field on GeneratedFile so the caller names the target explicitly. Either is additive.