feat(testing): add composable failure assertions #14

Summary

• Add failure queries that compose exact property paths with message, code, severity, and state filters.
• Add inverse filters and nonempty only assertions for complete result checks.
• Add sync and async test validation entry points with option selection.
• Update public examples and the source mirror.

Closes #13.

Validation

• rg --files src test -g '*.pudu' | xargs pudu check
• pudu lint src test
• pudu fmt --check src test
• pudu test test — 16 suites, 130 assertions
• pudu build src/Main.pudu -o /tmp/pudu-lang-validator
• Compiled CLI --version smoke run
• Independent source and wiki review