BE-474: HashQL: Take into account terminator target eligibility in execution backend placement

.claude/skills/testing-hashql/references/mir-builder-guide.md

@@ -158,6 +158,7 @@ let body = body!(interner, env; [graph::read::filter]@0/2 -> Bool {
 | `x = tuple <a>, <b>;`           | Create tuple          | `Assign(x, Aggregate(Tuple, [a, b]))`                |
 | `x = struct a: <v1>, b: <v2>;`  | Create struct         | `Assign(x, Aggregate(Struct, [v1, v2]))`             |
 | `x = closure <def> <env>;`      | Create closure        | `Assign(x, Aggregate(Closure, [def, env]))`          |
+| `x = opaque (<name>), <value>;` | Create opaque wrapper | `Assign(x, Aggregate(Opaque(name), [value]))`        |
 | `x = bin.<op> <lhs> <rhs>;`     | Binary operation      | `Assign(x, Binary(lhs, op, rhs))`                    |
 | `x = un.<op> <operand>;`        | Unary operation       | `Assign(x, Unary(op, operand))`                      |
 | `x = input.load! "name";`       | Load required input   | `Assign(x, Input(Load { required: true }, "name"))`  |
@@ -276,6 +277,27 @@ let body = body!(interner, env; [graph::read::filter]@0/2 -> Bool {
 });
 ```
 
+### Opaque Construction and Projection
+
+Construct opaque-wrapped values with `opaque (<name>), <value>`. The name must
+be wrapped in parentheses because it is a multi-token path.
+
+```rust
+use hashql_core::symbol::sym;
+
+let body = body!(interner, env; fn@0/0 -> Int {
+    decl inner: (x: Int, y: Int), wrapped: [Opaque sym::path::Entity; ?], result: Int;
+    @proj y_field = wrapped.y: Int;
+
+    bb0() {
+        inner = struct x: 100, y: 200;
+        wrapped = opaque (sym::path::Entity), inner;
+        result = load y_field;
+        return result;
+    }
+});
+```
+
 ### Direct Function Calls
 
 Use a `DefId` variable directly: