Add differential testing: Crane C++ vs Rocq OCaml extraction

[CharlesCNorton]

Summary

• Adds OCaml extraction to 3 existing test .v files (ack, module, mutual_mod) alongside their Crane C++ extraction
• Each test gets a pair of thin diff harnesses (.d.cpp / .d.ml) that print identical canonical output
• A driver script (scripts/run-diff-tests.sh) compiles both sides, runs both, and diffs stdout
• Reuses existing test definitions — no duplicated Coq code

What each test exercises

Test	Features
ack	Deep recursion, nat→int mapping
mutual_mod	Mutually recursive inductives, constant extraction
module	Functors, BST tree, option type, comparison

How to run

eval $(opam env --switch=default --set-switch)
dune build tests/basics
bash scripts/run-diff-tests.sh

Test plan

• All 3 differential tests produce identical C++ and OCaml output
• Existing tests still build and pass alongside the new OCaml extraction
• Driver script runs clean end-to-end