fix: preserve Scratch arithmetic semantics in identity folding

[HimanshuDeshbhratar]

Summary

Arithmetic identity optimizations in Pass 2 can silently change the behavior of valid Scratch programs.

The optimizer currently rewrites expressions such as:

• 0 + x → x
• x - 0 → x
• x * 1 → x
• x / 1 → x
• 0 * x → 0

without proving that the operands are numeric.

Scratch arithmetic uses Cast.toNumber, so these transformations are not generally semantics-preserving. For example, "apple" is converted to 0 in arithmetic.

Example

onflag {
    x = "apple";
    say 0 + x;
    say x - 0;
    say x * 1;
    say x / 1;
}
```goboscript

###Tests

Added regression tests covering:

-arithmetic identities with dynamic operands
-constant folding with Scratch Cast.toNumber semantics

Validation performed:

-cargo test
-cargo clippy --all-targets
-all existing tests/* projects compile successfully