feat(validation): compute failure metadata #12

Summary

• Derive failure severity and state from the source and selected property in typed rules.
• Apply equivalent root-based metadata to asynchronous rules after failures are produced.
• Make later static or computed setters take precedence without running replaced callbacks.
• Keep source mirrors and README in step with the public API.

Closes #11.

Validation

• rg --files src test -g '*.pudu' | xargs pudu check
• pudu lint src test
• pudu fmt --check src test
• pudu test test — 15 suites, 110 assertions
• git diff --check
• Independent source and wiki review