ocs_doc_api: dissolve src/app/doc_api_convert.rs shim into direct crate usage

Delete the thin re-export file; src/app/doc_api.rs now uses ocs_doc_api::convert
directly (use ocs_doc_api::convert + convert::* call sites). Remove the
mod doc_api_convert registration from src/app/mod.rs.

The conversion logic already lives in the crate (src/convert.rs) from the prior
move; the shim was redundant. 623 host lib + 38 doc_api tests green.

src/app/mod.rs

@@ -13,8 +13,6 @@ pub(crate) mod helpers;
 mod history;
 #[cfg(not(target_arch = "wasm32"))]
 mod doc_api;
-#[cfg(not(target_arch = "wasm32"))]
-mod doc_api_convert;
 mod layers;
 mod model_ops;
 mod mtext_editor;