Add k6 activity ramp benchmark

[rgarcia]

Summary

• add a k6 TypeScript activity-ramp benchmark for Hypeman instance lifecycle load
• add hypervisor selection and tagging for cloud-hypervisor, firecracker, and qemu matrix runs
• add a Make target that emits the local k6 HTML report and JSON summary with 120s dashboard buckets
• make benchmark setup compatible with image names containing slashes and preserve unique instance suffixes under long run IDs
• record create admission rejections separately with a short backoff so capacity limits do not look like script failures

Validation

• docker run --rm -v "$PWD":/src -w /src grafana/k6:latest inspect benchmarks/k6/activity-ramp.ts
• make -n bench-activity-ramp HYPEMAN_API_KEY=dummy
• ran 24 matrix benchmark runs across m8i host sizes, gp3 settings, and cloud-hypervisor/firecracker/qemu
• verified no active benchmark stacks remained after the matrix

CloudFormation quickstart options for ingress and gp3 tuning now live in PR #237.