- levels.test.ts: change length assertion to 16, add newKeys tests for levels 7-16
- levels.test.ts: add Level 16 allKeys completeness test (all QWERTZ keys)
- words.test.ts: extend all loops from 6 to 16 levels, add validateWordList test
- companion.test.ts: fix unknown-letter test to use '@' (z now has a fallback entry)