spec: KECCAK accelerator

[erik-3milabs]

This PR adds:

1. specification for a keccak accelerator
2. spec support for modulus notation with a constant modulus
3. spec support for multi-dimensional arrays

[github-actions]

Codex Code Review

1. Low (Potential bug): mod expression support is inconsistent between checker and renderer

• The checker accepts any constant expression as modulus (e.g. ["+", 4, 4]) in chip.py and builds it in chip.py.
• The Typst renderers require the modulus to be a literal int via type(e.at(2)) == int in expr.typ and expr.typ.
• Result: valid expressions for tooling/typecheck can fail during spec rendering.

No concrete security vulnerabilities, significant performance issues, or additional high-confidence bugs found in this PR diff.

[RobinJadoul]

Some comments and questions.
Confidence in several of them is fairly low, so feel free to tell me if I'm wrong somewhere :)