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/main.zig

@@ -722,6 +722,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
         // slices reference compileWithTimer locals that outlive the callback.
         e.build_config.c_object_paths = c_obj_paths;
         e.build_config.link_libraries = libs;
+        e.build_config.object_path = obj_path;
         // Android-specific bundling state.
         if (e.build_config.manifest_path == null) e.build_config.manifest_path = merged_config.manifest_path;
         if (e.build_config.keystore_path == null) e.build_config.keystore_path = merged_config.keystore_path;