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_doc_api/src/id.rs

@@ -0,0 +1,40 @@
+//! Stable entity identity (plan §2 decision #7).
+
+use serde::{Deserialize, Serialize};
+
+/// A stable u64 handle to a first-level document entity, decoupled from any
+/// internal engine/kernel pointer. Value-compatible with `acadrust::Handle` —
+/// kernel keys never cross the API boundary.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct ObjectId(pub u64);
+
+impl ObjectId {
+    /// The null / absent handle (`acadrust::Handle::NULL` value `0`).
+    pub const NULL: ObjectId = ObjectId(0);
+
+    pub const fn as_u64(self) -> u64 {
+        self.0
+    }
+
+    pub const fn is_null(self) -> bool {
+        self.0 == 0
+    }
+
+    /// Build from a raw u64 (e.g. an `acadrust::Handle`'s value).
+    pub const fn from_u64(v: u64) -> Self {
+        Self(v)
+    }
+
+    /// Build from an `acadrust::Handle` (host/ipc feature).
+    #[cfg(any(feature = "host", feature = "ipc", test))]
+    pub fn from_handle(h: acadrust::Handle) -> Self {
+        Self(h.value())
+    }
+
+    /// Convert to an `acadrust::Handle` (host/ipc feature).
+    #[cfg(any(feature = "host", feature = "ipc", test))]
+    pub fn to_handle(self) -> acadrust::Handle {
+        acadrust::Handle::new(self.0)
+    }
+}