feat: establish cross-platform validator parity core

[anusbutt]

Outcome

Describe the user-visible result and the smallest implementation boundary.

Evidence

List the failing test observed before implementation and the passing commands afterward.

Checklist

• Tests cover valid, invalid, and relevant security or performance paths.
• Documentation and examples reflect every changed public behavior.
• Schema compatibility is unchanged, additive and documented, or explicitly marked breaking.
• Constitution principles and append-only/evidence guarantees remain satisfied.
• npm run build passes.
• npm run typecheck passes.
• npm test passes with no hidden skips.
• npm run package:smoke passes and the tarball contains intended assets only.
• sh -n scripts/validate.sh and sh -n templates/pre-commit pass when shell files change.
• No secrets, credentials, telemetry, generated archives, or unrelated edits are included.

Compatibility and recovery

State supported platforms tested, rollback behavior, diagnostic changes, and any
manual recovery a repository owner would need.