Every stack movement now goes through pushv/popv and increments a depth
word on the function context. A call pads to 16 from wherever the expression
evaluator has left rsp, and asserts the parity before it emits the call.
The nested probe passes.
The stack-argument path is folded into the same counter rather than keeping
its own, because two independent notions of parity is how the bug comes back.
x86.ml is an instruction selector for the part of Tast that fits in one
integer register: literals, slots, let, if, arithmetic, comparison, and a
call. Everything else raises with the node that defeated it, because an
honest refusal is the measurement and a silently wrong answer would waste
the exercise.
The frontend is the real one -- Reader, Parse, Load, Check -- so what is
lowered is the same Tast.fn the LLVM backend gets. Seven arithmetic results
are compared against what the language says they should be; the disassembly
proves nothing and is not the evidence.
Nothing is wired into the build. No dune file under spike/, driven by hand
with ocamlfind and clang as spike/embed already does.
