T2: Create Validators module with Luhn and SSN validation

[dpaluy]

Task Details

Priority: p1 | Story Points: 2 | Group: A (Foundation)
Depends on: none

Files

• lib/tracebook/redactors/validators.rb
• test/lib/redactors/validators_test.rb

Acceptance Criteria

• Validators.luhn method validates credit card checksums
• Validators.ssn_range method validates SSN ranges (rejects 000, 666, 900+)
• Test vectors pass (Visa, MC, Amex, invalid prefixes)

Context

Part of PII Redaction Architecture Refactor - Phase 1: Core Infrastructure

See full plan: docs/plans/260106-02b-pii-redaction-architecture.md