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/gen/layouts.json

@@ -0,0 +1,28 @@
+{
+  "ObjectId": { "struct": [ { "name": "0", "type": "u64" } ] },
+  "Operation": {
+    "enum": [
+      "CreateSolid(SolidPrimitive)",
+      "CreateCurve(Curve2Spec)",
+      "Extrude { profile: ObjectId, direction: [f64; 3] }",
+      "Revolve { profile: ObjectId, axis: ([f64; 3], [f64; 3]), angle: f64 }",
+      "Transform { id: ObjectId, placement: PlacementSpec }",
+      "SolidBoolean { op: BoolOp, a: ObjectId, b: ObjectId, erase_sources: bool }",
+      "AddVertex { id: ObjectId, at: usize, point: [f64; 3] }",
+      "Delete { id: ObjectId }",
+      "CreateMany(Vec<EntitySpec>)",
+      "TransformMany { ids: Vec<ObjectId>, placement: PlacementSpec }",
+      "DeleteMany(Vec<ObjectId>)"
+    ]
+  },
+  "Query": {
+    "enum": [
+      "GetEntity { id: ObjectId }",
+      "GetBounds { id: ObjectId }",
+      "GetCentroid { id: ObjectId }",
+      "GetVolume { id: ObjectId }",
+      "GetIntersects { a: ObjectId, b: ObjectId }",
+      "GetGeometryRevision"
+    ]
+  }
+}