fix(typechecker): validate arrays.reduce callback return type matches accumulator (#2241)

[SchoolyB]

Fixes #2241

arrays.reduce() was missing validation that the reducer callback's return type matches the accumulator type. When a callback returned a different type (e.g., string instead of int), the typechecker accepted it and codegen produced code that reinterpreted the return value's bits as the accumulator type, resulting in garbage data at runtime.

Added an else if branch in the existing reduce callback validation (E9004) that resolves the initial value's type and compares it against the callback's return type, emitting E9004 on mismatch.

Added fail test E9004_arrays_reduce_wrong_return.gray.