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/ipc/protocol.rs

@@ -138,6 +138,12 @@ 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`.
@@ -164,6 +170,10 @@ 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 `Receipt` 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.