feat(async): add validator prevalidation #22

Summary

• Add synchronous and awaited prevalidation hooks to async validators.
• Return early success or failures before selecting and awaiting ordinary rules.
• Preserve root context and prefix child prevalidation paths when the child rule is selected.
• Cover replacement, selection, collection indices, and continuation behavior.

Closes #21.

Validation

• rg --files src test -g '*.pudu' | xargs pudu check
• pudu lint src test
• pudu fmt --check src test
• pudu test test — 20 suites, 184 assertions
• pudu build src/Main.pudu -o /tmp/pudu-lang-validator
• Compiled CLI --version smoke run
• Independent source and wiki review