docs(blake3): record XOF output boundary

[brenorb]

Summary

• add a deterministic probe showing the independent BLAKE3 XOF can produce 64 bytes for a 32-byte message
• record that the local generators expose only the unkeyed 32-byte output contract
• add NR-046 and OP-022 with the missing output-block schedule and acceptance criteria

Validation

• cargo run --locked --example blake3_xof_boundary
• cargo test --locked --lib hashes::blake3::tests::test_official_test_vectors
• python3 tools/kb.py validate
• cargo fmt --check
• git diff --check

The full repository test suite was not run; this is a scoped research/documentation contribution.