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/error.rs

@@ -0,0 +1,59 @@
+//! Structured error model (plan §2 decision #9). Serializable so IPC returns the
+//! same error the in-process executor produced.
+
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+
+use crate::id::ObjectId;
+
+/// Which kind of kernel failure occurred (a mapped `cadkernel` `Snag`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum GeometryErrorKind {
+    /// The operation produced an empty / degenerate body.
+    Empty,
+    /// Boolean refused (coincident geometry, unsupported cut, …).
+    BooleanFailed,
+    /// Invalid input to a kernel constructor (e.g. non-positive radius).
+    InvalidInput,
+    /// ACIS lift/parse/lower failed.
+    Acis,
+    /// Any other kernel error (message carries detail).
+    Other,
+}
+
+/// The single error type for the whole API. `commit`/ops return
+/// `Result<_, ApiError>`; on a failed op nothing is applied and no undo step is
+/// recorded (per-op atomicity).
+#[derive(Error, Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum ApiError {
+    /// Input failed validation before any mutation (names the op + reason).
+    #[error("validation failed on {op}: {reason}")]
+    Validation { op: String, reason: String },
+
+    /// The geometry kernel reported a failure (a mapped `Snag`).
+    #[error("geometry kernel: {kind:?}: {msg}")]
+    Geometry { kind: GeometryErrorKind, msg: String },
+
+    /// An `ObjectId` does not resolve to a live entity (stale/deleted/never existed).
+    #[error("unknown ObjectId {0:?}")]
+    UnknownId(ObjectId),
+
+    /// The requested capability is not supported (e.g. over-cap bulk op, unimplemented family).
+    #[error("unsupported capability: {0}")]
+    Unsupported(String),
+
+    /// The transport (IPC / channel) failed — disconnected, oversized, timeout.
+    #[error("transport: {0}")]
+    Transport(String),
+}
+
+pub type ApiResult<T> = Result<T, ApiError>;
+
+impl ApiError {
+    pub fn validation(op: &'static str, reason: impl Into<String>) -> Self {
+        ApiError::Validation { op: op.to_string(), reason: reason.into() }
+    }
+    pub fn geometry(kind: GeometryErrorKind, msg: impl Into<String>) -> Self {
+        ApiError::Geometry { kind, msg: msg.into() }
+    }
+}