Add Testcontainers spike document and proof of concept

Spike investigation for replacing Makefile + Docker setup with Testcontainers:

Benefits:
- Pure Go solution (no shell scripts, no Makefile)
- Automatic container lifecycle management
- Built-in readiness checks (no manual wait loops)
- Parallel test execution support
- Better error messages and debugging
- Simpler CI/CD (just 'go test')

Includes:
- Comprehensive spike document with examples
- Proof of concept helper code
- Migration strategy
- Performance comparison
- Trade-offs analysis

This is exploratory - not ready for production use yet.

Author: zph