Add kernel solid operations and complete Solid3D (create and rigid move)

An additive HostApi::solid_operation builds cadkernel primitives and applies
rigid transforms, verifying each result lifts back losslessly and refusing a
payload it cannot lift. Exposed as doc.solids in Python; Solid3D, Body and
Region are mapped read-only with the opaque payload preserved. The lifecycle
test passes through the real runner with analytic volumes and exact extents as
oracles, DWG and DXF reopen with re-edit, and undo restores exactly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/ocs_plugin_api/src/ipc/protocol.rs

@@ -149,6 +149,8 @@ pub enum PluginRequest {
     /// V7: synchronous selection read/write for the dispatch tab.
     GetSelection,
     SetSelection { handles: Vec<Handle> },
+    /// V7 (additive): kernel-backed solid create or transform.
+    SolidOperation { operation: crate::host::SolidOperation },
 }
 
 /// Responses the host sends back for `PluginRequest`.
@@ -180,6 +182,7 @@ pub enum PluginResponse {
     EntityTransactionResult(Result<(), String>),
     Selection(Vec<Handle>),
     SelectionResult(Result<(), String>),
+    SolidResult(Result<Handle, String>),
 }
 
 /// Messages sent from the host to the plugin runner.