Add kernel booleans: union, subtract and intersect for Solid3D

SolidOperation::Boolean runs synchronously, surfaces the kernel's refusal
with a plain reason and leaves the operands untouched, consumes them on
success unless keep_operands, and records one undo step. The lifecycle test
uses planar boxes with exact volumes and covers chaining, refusals, both
formats and undo; an ignored test shows a curved refusal reaching the script.

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

crates/ocs_plugin_api/src/host.rs

@@ -76,6 +76,15 @@ pub enum SolidPrimitive {
     Pyramid { center: [f64; 3], radius: f64, height: f64, sides: u32 },
 }
 
+/// How two solids combine.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum SolidBoolean {
+    Union,
+    /// Remove the second solid from the first.
+    Subtract,
+    Intersect,
+}
+
 /// A kernel operation on ACIS-backed solids (API v7, additive). The host owns
 /// the geometry: a script never sees or rewrites the payload.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
@@ -95,6 +104,18 @@ pub enum SolidOperation {
     /// Extrude a planar profile along `direction`: a solid when the profile is
     /// closed, a surface when it is open.
     Extrude { source: Handle, direction: [f64; 3], layer: Option<String>, delete_source: bool },
+    /// Combine two solids into a new one. Both operands are consumed unless
+    /// `keep_operands`; the result goes on `layer` or the first operand's
+    /// layer. The kernel refuses some cases (coincident faces, cuts it has no
+    /// closed form for); a refusal changes nothing. Curved operands can take a
+    /// second or two, and the host thread is busy meanwhile.
+    Boolean {
+        first: Handle,
+        second: Handle,
+        operation: SolidBoolean,
+        layer: Option<String>,
+        keep_operands: bool,
+    },
 }
 
 /// Value of a host-managed drafting or document setting exposed to plugins.

crates/ocs_plugin_api/src/ipc/v4/protocol.rs

@@ -115,6 +115,13 @@ mod tests {
             },
             SolidOperation::Transform { handle: acadrust::Handle::new(7), matrix: [1.0; 16] },
             SolidOperation::RegionFromProfile { source: acadrust::Handle::new(3), layer: None, delete_source: true },
+            SolidOperation::Boolean {
+                first: acadrust::Handle::new(1),
+                second: acadrust::Handle::new(2),
+                operation: crate::host::SolidBoolean::Subtract,
+                layer: None,
+                keep_operands: false,
+            },
             SolidOperation::SurfaceFromProfile { source: acadrust::Handle::new(4), layer: Some("S".into()), delete_source: false },
             SolidOperation::Extrude { source: acadrust::Handle::new(5), direction: [0.0, 0.0, 2.0], layer: None, delete_source: true },
         ] {