Feature: add if/else conditional blocks

[nholthaus]

Problem

The scripting language supports counted loops, but it does not support conditional branching. This prevents scripts from expressing runtime decisions and forces control flow to remain linear.

Specification

Add support for conditional blocks in script files.

Acceptance criteria:

• Scripts can begin a conditional block with an if statement.
• Scripts can optionally provide an else branch.
• Scripts can terminate the conditional with a matching end_if marker.
• The condition can compare two operands using a small built-in comparison vocabulary.
• Nested control-flow blocks remain parseable and unambiguous.
• Invalid operators or unmatched blocks fail with a clear error message.

Motivation

This would enable scripts to branch based on runtime state instead of only executing fixed sequences of commands.