fix(codegen): check narrowed int32 operand

[1sgtpepper]

Closes #1182

int32_to_uint and try_int32_to_uint were duplicating the word below the value being narrowed before applying the unsigned range mask. With another live value below the operand, checked narrowing could reject a valid value or accept an out-of-range value based on the wrong stack word.

This changes both helpers to duplicate the top stack value before masking. The full-width n == 32 case uses a zero mask so the mask construction does not shift by 32.

Regression coverage compiles HIR snippets and executes them through the package evaluator:

• checked casts from u32 and i32 to i1, u8, and u16;
• overflowing u8 addition, which covers the try_int32_to_uint path;
• live guard values below the checked operand, so the old dup.1 behavior fails on both valid and invalid inputs.

[1sgtpepper]

@bitwalker Could you approve the CI run? Thanks!