feat: add ucp_validate.py CLI readiness validator

Standalone merchant readiness validator for UCP endpoints.
Point it at any merchant URL, get a structured pass/fail report.

Features:
- Fetches and validates /.well-known/ucp discovery profile
- Dual schema version support (2026-01-11 + 2026-01-23)
- Structural consistency checking (version vs service/capability format)
- Only checkout is required; other declared capabilities validated
- Payment handler validation (structure, reverse-DNS naming)
- URL reachability checking for all spec/schema references
- Signing key JWK format validation
- Optional checkout smoke test (--smoke)
- JSON output mode (--json)
- Colored terminal output with ANSI badges
- Exit code 0 for READY, 1 for NOT_READY

Usage:
  uv run ucp_validate.py https://merchant.example.com
  uv run ucp_validate.py http://localhost:8182 --smoke
  uv run ucp_validate.py https://merchant.example.com --json

Author: sbeashwar