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.

library/modules/std/build.sx

@@ -15,3 +15,8 @@
 // the linker. Answered from the compiler's accumulated build state.
 c_object_paths :: () -> List(string) abi(.compiler);
 link_libraries :: () -> List(string) abi(.compiler);
+
+// The object file the compiler emitted for this build. The compiler emits it
+// eagerly; this returns its path (a query, not an action). The sx driver passes
+// it to `link` alongside the C objects.
+emit_object :: () -> string abi(.compiler);