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/query_gen.rs

@@ -0,0 +1,22 @@
+// GENERATED by build.rs from spec/entities.toml — do NOT edit by hand.
+// Committed snapshot for reviewability (plan §10); build.rs regenerates and CI
+// fails on drift (`--check`). Append new variants at the END only (bincode
+// discriminant stability, plan §7).
+
+use serde::{Deserialize, Serialize};
+
+use crate::id::ObjectId;
+
+/// A typed read query (plan §5). Read-only: no mutation, no undo, no revision
+/// bump. Safe to batch (`EnvelopeBody::Queries`). Append new variants at the END only.
+#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
+#[non_exhaustive]
+pub enum Query {
+    GetEntity { id: ObjectId },
+    GetBounds { id: ObjectId },
+    GetCentroid { id: ObjectId },
+    GetVolume { id: ObjectId },
+    /// Whether two entities' bounds overlap (two-phase conditional modelling).
+    GetIntersects { a: ObjectId, b: ObjectId },
+    GetGeometryRevision,
+}