feat(u32): add checked byte-to-bit adapter

[brenorb]

Summary

• add u32_to_le_bits() for four checked byte limbs to 32 little-endian bit items
• reject numeric byte values outside 0..=255 and test all byte values plus boundary patterns
• record the 520-byte / 9-byte witness / 35-item peak / 370-opcode profile

Validation

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

The full repository test suite was not run.