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.lock

@@ -38,7 +38,6 @@ dependencies = [
  "objc2 0.5.2",
  "objc2-app-kit 0.2.2",
  "objc2-foundation 0.2.2",
- "ocs_doc_api",
  "ocs_plugin_api",
  "open",
  "printpdf",
@@ -3891,20 +3890,6 @@ dependencies = [
  "objc2-foundation 0.2.2",
 ]
 
-[[package]]
-name = "ocs_doc_api"
-version = "0.1.0"
-dependencies = [
- "acadrust",
- "bincode",
- "cadkernel",
- "ocs_plugin_api",
- "serde",
- "serde_json",
- "thiserror 1.0.69",
- "toml 1.1.3+spec-1.1.0",
-]
-
 [[package]]
 name = "ocs_plugin_api"
 version = "0.2.1"