Add ocs_doc_api: object-oriented CAD entity API (v2)

New crate crates/ocs_doc_api: object-centric facade (DocApi -> Document ->
collections -> typed handles), transport-agnostic wire layer (DocApiEnvelope =
single op | read-only query batch), per-op autocommit (no write transactions),
bulk ops with upfront-validation all-or-nothing, structured ApiError, and
build-time codegen (spec/entities.toml -> ops/query enums, api_reference.md,
binding_schema.json).

ocs_plugin_api: append-only PluginRequest::DocApiRequest / PluginResponse::
DocApiResponse (serde_bytes payload) + defaulted HostApi::doc_api_dispatch.

Host: DocApiBackend impl for HostSession (src/app/doc_api.rs) mapping to scene
primitives (solid_models lift-on-miss, register_prepared_solid_model, meshes
metrics), entity conversions (doc_api_convert.rs), HostSession accessors +
doc_api_dispatch override.

Tests: 592 host lib (incl. 7 DocApi integration tests: create/boolean/query,
unknown-id, transform-many all-or-nothing, query-batch cap, 2D/geometric/DWG
roundtrips), 8 crate unit tests. Examples + README/ARCHITECTURE/IMPLEMENTATION docs.

crates/ocs_plugin_api/src/host.rs

@@ -552,6 +552,18 @@ pub trait HostApi {
         let _ = tab_id;
         None
     }
+
+    /// DocApi v2 (`ocs_doc_api`): dispatch one bincode-serialized `DocApiEnvelope`
+    /// (one write op OR a read-only query batch) and return the bincode-serialized
+    /// `Receipt` (or the bincode-serialized `ApiError` on failure). Opaque bytes:
+    /// this crate does not depend on `ocs_doc_api`, it only routes the envelope.
+    ///
+    /// Added for the DocApi v2 adapter. The default returns `Err` ("not supported");
+    /// the in-process host (`HostSession`) overrides it to call the DocApi executor.
+    fn doc_api_dispatch(&mut self, tab_id: u64, bytes: &[u8]) -> Result<Vec<u8>, String> {
+        let _ = (tab_id, bytes);
+        Err("DocApi v2 not supported by this host".to_string())
+    }
 }
 
 /// Simplified, read-only entity kind exposed by [`DocumentReader`].