feat(u32): add conditional negation

[brenorb]

Summary

• add u32_conditional_negate for normalized sign routing over the byte-word representation
• accept any nonzero condition while preserving the zero branch
• cover boundary words and hostile non-boolean conditions
• record policy-produced script, stack, and static opcode metrics

Validation

• cargo test --locked arithmetic::u32::stack::tests::test_u32_conditional_negate
• cargo test --locked --test primitive_metrics
• python3 tools/kb.py validate
• cargo fmt --check
• git diff --check

The full repository test suite was intentionally not run; the focused primitive and metric checks pass. The local executor does not expose a useful dynamic opcode counter, so the documentation reports 52 static non-push opcodes instead.