docs(blake3): record keyed mode boundary

[brenorb]

Summary

• record that the local BLAKE3 generators expose unkeyed mode only
• add a deterministic keyed-vs-unkeyed boundary probe
• document the missing key-word/flag pricing frontier as NR-047 and OP-023

Validation

• cargo fmt --check
• git diff --check
• python3 tools/kb.py validate
• cargo run --quiet --example blake3_keyed_boundary
• cargo test --locked hashes::blake3::tests::test_official_test_vectors -- --exact

The full repository test suite was not run.