feat: add mod, a floored remainder that never follows a negative dividend

[VedantMadane]

Summary

% truncates toward zero (-7 % 3 is -1), which is the right division remainder but the wrong answer for wrapping. Every ring buffer / clock / hash-bucket use ends up written as ((n % d) + d) % d.

Change

• New builtin mod(n, d): floored remainder; sign follows the divisor.
• Zero divisor refuses with the same "modulo by zero" words as %.
• Integers stay integers; any float promotes, matching other binary numeric builtins.
• Spec section 8.7 documents mod and how it differs from %.
• Unit tests cover positive/negative divisors, floats, zero, and type errors.
• Builtin registry counts updated so the pinned-list guards stay honest.

Test plan

• cargo test --lib mod_
• cargo test --lib the_pinned_list
• cargo test --lib builtin_kind_counts

Fixes #44

[VedantMadane]

Closing as duplicate of #54 (same mod builtin).