feat(u32): add direct zero predicate

[brenorb]

Summary

• add u32_iszero() for the existing four-byte u32 representation
• fold four per-limb zero tests without constructing a second zero word
• document the canonical-byte precondition and measured boundary

Measurements

• 4 policy-compiled locking-script bytes
• 5 serialized bytes for the four-item zero witness
• 4-item peak and 4 static non-push opcodes
• 21-byte u32_push(0) + u32_equal() baseline

Validation

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

The full repository test suite was intentionally not run.