feat(assert): report missing arguments

[Chemaclass]

Summary

• report under-supplied core comparison assertions as test usage errors
• preserve explicit empty values, optional trailing arguments, and over-supplied calls
• route usage diagnostics through the existing Error channel without adding forks
• cover 21 core assertions: equality, containment/regex, prefix/suffix/format, numeric comparisons, delta, and line count

Closes #983

Validation

• make sa
• make lint
• ./bashunit --parallel --simple --strict tests/ (1,653 tests)
• bash build.sh bin -v (1,653 tests)