feat(async): compose child validators #6

Summary

• Compose required, optional, and indexed child validators across the async boundary.
• Preserve root context, selected rule sets, selected paths, and deterministic failure order.
• Add awaited conditions and custom failure callbacks.

Validation

• pudu check $(rg --files src test -g '*.pudu')
• pudu fmt --check src test
• pudu lint src test
• pudu test test — 11 suites, 71 assertions

Closes #5