fix(test): remove trailing semicolons in test_opt_if_fold if-branches

`tests/codegen/test_opt_if_fold.affine` had `if true { 42; } else { 0; }`
where the trailing `;` makes each branch return Unit. The recent
typechecker work (in lib/typecheck.ml on this branch) tightened
unification so this now produces:

    Unification error: (Unify.TypeMismatch (Unit, Int))
    affinescript: Type error

Removed the trailing semicolons inside the if-branches; bare expressions
now return Int and `let x = if ...` typechecks. Added a comment
documenting the gotcha for future test authors.

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

tests/codegen/test_opt_if_fold.affine

@@ -1,7 +1,11 @@
 // Test if-expression constant folding
 
 fn main() -> Int {
-  let x = if true { 42; } else { 0; };   // Should fold to 42
-  let y = if false { 99; } else { 10; }; // Should fold to 10
+  // No trailing semicolons inside the if-branches: a trailing `;` makes
+  // the branch return Unit, so `let x = if ...` would fail with
+  // "Unification error: (Unify.TypeMismatch (Unit, Int))". The bare
+  // expression form returns Int.
+  let x = if true { 42 } else { 0 };   // Should fold to 42
+  let y = if false { 99 } else { 10 }; // Should fold to 10
   return x + y;  // 42 + 10 = 52
 }