Function apply_companions 

pub fn apply_companions(
    files: &mut Vec<GeneratedFile>,
    companions: Vec<GeneratedFile>,
)

Merge downstream Companion files into the per-package stitcher produced by generate.

For each companion file this function locates the PackageMod entry in files with a matching package and appends include!("<name>"); at file scope after buffa’s own output — at package root, alongside the owned message types, not under __buffa::. The companion files themselves are appended to files so that build integrations can write everything to disk in one pass.

Call this once per build; it does not deduplicate, so a second call with the same companions emits a second include! for each, which fails to compile downstream with a duplicate-definition error.

name must be a bare-sibling filename — the same convention buffa uses for its own include! calls, so it resolves relative to the stitcher without any OUT_DIR prefix. Names must not contain ", \, /, or newlines (the function debug_assert!s this in debug builds), and must not collide with any of buffa’s own generated filenames for the same package (<stem>.rs, <stem>.__view.rs, etc.) — pick an unused suffix such as <stem>.__myplugin.rs.

Companion files with no matching PackageMod (e.g. for a package buffa did not generate any output for) are still appended to files but no include! is emitted; the caller is responsible for wiring them up. If you don’t expect orphans, check that every companion’s package appears in files as a PackageMod after calling.