Validate direct workload operators

[Anionix]

Summary

• reuse the centralized is_comparison_operator guard inside WorkloadSpec.validate()
• keep direct construction fail-closed even when untyped callers bypass the public Literal
• add a direct-construction regression for the source review defect

Evidence

• PYTHONPATH=src /Users/st/Documents/data-format-lab/.venv/bin/pytest tests/test_model.py (23 passed)
• uvx ruff check src/format_bench/model.py tests/test_model.py (passed)
• uvx pyright (0 errors, 0 warnings)
• independent code-review skill: Standards 0 findings; Spec 0 findings at 17a184701dea74bc52e7b4ac9e0002672f9523a1
• source PR Type adapter workload contracts #501 thread was replied with bug [bug] Keep direct WorkloadSpec operator validation fail-closed #504 and resolved
• NO_LEAN: no Lean sources or theorem boundary changed

Fixes #504