Update keccak HWSL constraints to equivalent arith constraints

[RobinJadoul]

Bringing the findings from #860 cleanly into the spec

[github-actions]

Kimi Code Review

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Automated review by Kimi (Moonshot AI)

[github-actions]

Codex Code Review

• Medium – Constant exponents are incorrectly validated as zero (chip.py): Every constant variable is inserted into valmap as zero. Consequently, 2^rnc[x][y] is type-checked as 2^0, masking the non-literal exponent instead of validating the actual fixed value. This can allow invalid constraint expressions to pass. Populate real constants or explicitly support symbolic fixed-column exponents.

• Low – Chip.typecheck() depends on an unrelated global (chip.py): Environment(config, ...) references the CLI-only global config; imported/library use raises NameError. Use self.config.