Fix unsigned integer underflow in help formatting

[metsw24-max]

This patch fixes multiple unsigned integer underflow issues in the help generation logic.

Fix

All affected arithmetic operations are now guarded:

• Subtractions are clamped to 0 when underflow would occur
• Multi-term expressions are validated before subtraction
• Fix applied consistently across all Wrap call sites

[Taywee]

Looks good to me. I'll move the test_underflow file into ./test after merge. Thanks for the PR.

edit: Actually, that's just used for buck. I'll leave it where it is.