Support Erlang/OTP 29 builds

[sgiath]

Summary

• fix native include/lib path discovery on Erlang/OTP 29 by using ~s in the Makefile erl -eval expressions
• update CI and docs to cover Erlang/OTP 27, 28, and 29

Root cause

OTP 29 rejects the old ~ts formatting expressions used by c_src/Makefile, so the shell commands returned no usable include paths. The native compiler then ran without the ERTS include directory and failed to find erl_nif.h.

Validation

• nix shell nixpkgs#erlang_29 nixpkgs#gcc --command make -C c_src -B V=1
• nix shell nixpkgs#erlang_28 nixpkgs#gcc --command make -C c_src -B V=1
• nix shell nixpkgs#erlang_27 nixpkgs#gcc --command make -C c_src -B V=1
• OTP 29 smoke test loaded the NIF and produced the expected 64-byte scrypt vector

Fixes #9

[kpy3]

Thanks for your PR!