P5.2: emit_object() -> string query primitive

The compiler emits the sx object eagerly (the Zig driver, before the post-link
callback), so emit_object is a QUERY (not an action): it returns the path from
a new BuildConfig.object_path field main.zig forwards — no driver vtable. This
completes the build-pipeline QUERY primitives (emit_object / c_object_paths /
link_libraries); only link (the genuine action) remains for the vtable step.

Extended examples/1662 to also assert emit_object().len > 0. 703/0 both gates.

src/ir/comptime_vm.zig

@@ -1469,6 +1469,17 @@ pub const Vm = struct {
                 return self.failMsg("comptime link_libraries: no build config threaded into the VM");
             return try self.makeStringList(table, result_ty, bc.link_libraries);
         }
+        // `emit_object() -> string` — the compiler emits the object eagerly (the Zig
+        // driver, before the post-link callback); this returns its path. A query,
+        // not an action — so no driver callback is needed (unlike `link`).
+        if (std.mem.eql(u8, name, "emit_object")) {
+            if (args.len != 0) return self.failMsg("comptime emit_object: expected no args");
+            const bc = self.build_config orelse
+                return self.failMsg("comptime emit_object: no build config threaded into the VM");
+            const path = bc.object_path orelse
+                return self.failMsg("comptime emit_object: no object was emitted (object_path unset)");
+            return try self.makeStringValue(table, path);
+        }
         return null; // not a known compiler function → caller bails to legacy
     }