Feature: add while loop support

[nholthaus]

Problem

The scripting language supports counted for loops, but it does not support condition-based repetition. This makes it awkward or impossible to express scripts that need to keep running until a runtime condition changes.

Specification

Add support for a while loop construct in script files.

Acceptance criteria:

• Scripts can begin a condition-based loop with a while statement.
• Scripts can terminate that loop with a matching end marker.
• The loop condition is evaluated on each iteration.
• Nested loop/control-flow blocks remain parseable and unambiguous.
• Invalid or unmatched loop blocks fail with a clear error message.

Motivation

This would allow scripts to model open-ended repetition driven by runtime state rather than fixed iteration counts.