[WIP] Fix test for signed/unsigned comparison semantics

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

---

This section details on the original issue you should resolve

<issue_title>Fix, test signed/unsigned comparison semantics</issue_title>
<issue_description>There's the classical C footgun:

int x = -1;
unsigned int y = 1;
if (x < y) { ... }  // branch NOT taken — x converts to UINT_MAX

We're currently subject to this behavior, which isn't consistent with Python semantics (which we otherwise try to follow). Tweak map_comparison in ExpressionToCExpressionMapper in loopy/target/c/codegen/expression.py to treat mixed-signedness comparisons correctly. Add exhaustive tests.</issue_description>

Comments on the Issue (you are @copilot in this section)