bugc: add the % operator

[gnidan]

BUG had no way to take a remainder. The IR already had a mod binary op, the EVM generator already mapped it to MOD, and constant folding already handled it, so only the front end was missing. This adds % to the parser, at the same precedence as * and /, and lets the typechecker accept it on numeric operands. IR generation already mapped % to mod.

BUG has no signed integer types, so MOD is the only opcode involved; there is no SMOD case. A zero divisor behaves as it does for /: the constant folder leaves x % 0 alone, and at run time the EVM's MOD returns 0.

Tests cover parsing and precedence, typechecking, IR generation, constant folding, and a behavioral test that compiles and runs programs at optimization levels 0 through 3 (1071 % 462 is 147, and x % 0 is 0, as is x / 0).

[github-actions]

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2026-10-04 18:16 UTC