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.

Cargo.toml

@@ -11,7 +11,6 @@ members = [
     "crates/ocs_web_worker",
     "crates/dwg-thumbnailer",
     "crates/dwg-thumbnailer-win",
-    "crates/ocs_doc_api",
 ]
 
 [target.'cfg(windows)'.build-dependencies]
@@ -76,7 +75,6 @@ iced_widget = { git = "https://github.com/iced-rs/iced.git", rev = "23604ff22ab0
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 getrandom = "0.3"
 ocs_plugin_api = { path = "crates/ocs_plugin_api", features = ["host"] }
-ocs_doc_api = { path = "crates/ocs_doc_api", features = ["host", "ipc"] }
 bincode = "1.3"
 meshopt = "0.6.2"
 i18n-embed = { version = "0.16", features = ["desktop-requester"] }