_encode_call() owns the cursor assignment and the _force suffix, so
the convention lives in one place instead of at every emission site;
the fixed32 fast path is an arm of the generic encode_content keyed by
a per type value template. write_fixed32_le uses convert_little_endian
instead of its own byte order switch. The integration test shares a
StateWaiter from state_utils and leaves the disconnect to the fixture.