docs: add Phase 0 environment assessment and test baseline (#1)

## Summary

Complete Phase 0 assessment of core-commerce package with comprehensive
architecture review, security analysis, and test baseline documentation.

## Key Findings

- **Architecture:** Mature Laravel package with event-driven design
- **Services:** 16 singleton services, dual payment gateway support (Stripe + BTCPay)
- **Security:** Recent P1/P2 security enhancements completed (webhook idempotency,
  per-IP rate limiting, fraud detection)
- **Testing:** Pest v3 framework with 14 test files (cannot execute due to
  private dependency on host-uk/core)
- **Codebase:** 185 PHP files, 32 models, 27 services (~8,434 LOC), strict typing

## Status

⚠️ Cannot execute tests/lint/analysis without access to private `host-uk/core` dependency.
Created FINDINGS.md with detailed assessment and recommendations.

Co-Authored-By: Clotho <clotho@lthn.ai>

Author: Clotho