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/manifest.rs

@@ -18,15 +18,9 @@
 /// plugins and appends `document_reader` / `document_view` at the end of the
 /// vtable so API v2 plugins keep working. v4 adds full-duplex notifications on
 /// a multiplexed socket while leaving the V2/V3 ABI and protocol untouched. v5
-/// adds the `BuiltinPlugin::on_load` lifecycle callback.
-/// Host ABI major version. Bumped to 6 when the DocApi v2 surface
-/// (`PluginRequest::DocApiRequest` / `PluginResponse::DocApiResponse` +
-/// `HostApi::doc_api_dispatch`) was added: plugins that call DocApi require an
-/// API-6 host and are rejected at handshake by older (API ≤ 5) hosts, instead of
-/// failing at runtime with a bincode-decode transport error. The DocApi wire
-/// changes are append-only and `doc_api_dispatch` is a defaulted method, so
-/// older V2–V5 plugins still load unchanged on an API-6 host.
-pub const API_VERSION: u32 = 6;
+/// adds the `BuiltinPlugin::on_load` lifecycle callback and
+/// `HostApi::document_path`.
+pub const API_VERSION: u32 = 5;
 
 /// Oldest plugin API major the current host still loads. This keeps previously
 /// compiled cdylibs usable as long as their vtable layout is a prefix of the