throwaway: atgc stack workflow testing

feat(scripts): add count-fns.sh with a test and docs #192

targeting e2e/derivative-kopis-target from e2e/derivative-kopis-feature
Merged opened by and assigned to derivative-kopis.test.pds.did.bot
targeting e2e/derivative-kopis-target from e2e/derivative-kopis-feature

Adds scripts/count-fns.sh, which prints how many fn items each Rust file defines (including pub(crate), async, const and unsafe fns), then a total. scripts/test-count-fns.sh checks it against a fixture tree, and the README says how to run both.

Closes #191

fn count per file

Commits 3

docs: describe the scripts in the README
test(scripts): check count-fns.sh against a fixture tree
feat(scripts): count fn items per Rust file