remove ocs_doc_api crate and DocApi v2 host integration

- Drop crates/ocs_doc_api from workspace and host dependency.
- Remove src/app/doc_api.rs and its module declaration.
- Strip DocApi v2 undo/dispatch plumbing from HostSession.
- Remove HostApi::doc_api_dispatch default method.
- Remove PluginRequest::DocApiRequest / PluginResponse::DocApiResponse.
- Revert plugin API version from 6 back to 5.

src/app/mod.rs

@@ -9,8 +9,6 @@ pub(crate) mod config;
 pub use automation::{export_headless, serve};
 mod command_driver;
 pub(crate) mod commands;
-#[cfg(not(target_arch = "wasm32"))]
-mod doc_api;
 mod document;
 pub(crate) mod expr_eval;
 mod find_replace;