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.

crates/ocs_plugin_api/src/ipc/protocol.rs

@@ -138,12 +138,6 @@ pub enum PluginRequest {
     GetTabId,
     /// V5: ask the host for the filesystem path of the document in `tab_id`.
     DocumentPath { tab_id: u64 },
-    /// DocApi v2 (`ocs_doc_api`): carry one bincode-serialized `DocApiEnvelope`
-    /// (one write op OR a read-only query batch). Appended at the end to keep
-    /// existing discriminants stable. The host routes this to the DocApi executor.
-    /// `serde_bytes` keeps the payload a single length-prefixed bulk copy (the
-    /// outer frame would otherwise walk it byte-by-byte).
-    DocApiRequest { tab_id: u64, #[serde(with = "serde_bytes")] bytes: Vec<u8> },
 }
 
 /// Responses the host sends back for `PluginRequest`.
@@ -170,10 +164,6 @@ pub enum PluginResponse {
     TabId(u64),
     /// V5: filesystem path of the document in the requested tab, if any.
     DocumentPath(Option<std::ffi::OsString>),
-    /// DocApi v2 (`ocs_doc_api`): the bincode-serialized `Result<Receipt, ApiError>` answering a
-    /// `PluginRequest::DocApiRequest`. Appended at the end (discriminant stability).
-    /// `serde_bytes` keeps the payload a single length-prefixed bulk copy.
-    DocApiResponse { #[serde(with = "serde_bytes")] bytes: Vec<u8> },
 }
 
 /// Messages sent from the host to the plugin runner.

crates/ocs_plugin_api/src/ipc/server.rs

@@ -77,9 +77,5 @@ pub fn handle_plugin_request(
         DocumentPath { tab_id } => PluginResponse::DocumentPath(
             host.document_path(tab_id).map(|path| path.into_os_string()),
         ),
-        DocApiRequest { tab_id, bytes } => match host.doc_api_dispatch(tab_id, &bytes) {
-            Ok(bytes) => PluginResponse::DocApiResponse { bytes },
-            Err(msg) => PluginResponse::Error(msg),
-        },
     }
 }