Bug: invalid for-loop bounds produce cryptic 'stoll' error

[nholthaus]

Problem

When a for loop receives a non-integer start or end value, the runtime currently surfaces a cryptic internal parsing message such as FATAL ERROR: 'for' stoll. This leaks an implementation detail instead of telling the script author what is wrong with the loop input.

Specification

Improve the user-facing error for invalid for loop bounds.

Acceptance criteria:

• Non-integer start or end values for for fail with a clear, user-meaningful error message.
• The error explains that the loop bounds must be valid integers.
• The error still includes normal source location context.
• Valid for loops keep their current behavior.

Motivation

This is a small correctness-and-polish bug that improves debuggability for script authors without changing the language surface.