[RobGrafrath]

What this changes and why

This records two decisions Rob made on 2026-10-10: the first production-scale test uses a 32 vCPU host, and a host is kept for as long as a test campaign needs it instead of being torn down and rebuilt for every run, since a rebuild repeats work that already exists.

• README: the principle "Nothing stays up" becomes "Nothing stays up without an end date".
• docs/workload-shapes.md: adds the 32 vCPU shape (m6id.8xlarge) and replaces the sentence saying the stack assumes a rebuild.
• docs/run-a-benchmark.md: adds a section, "Keeping a host for a campaign", and a pointer from the introduction.
• docs/cost-guardrails.md: points to that section from the timer layer.

Does it make a host easier to leave running or easier to reach?

Yes, on purpose, because it documents keeping a host up for a campaign. The controls stay in place. The lifetime timer is still required and is set to the campaign's length, the end date goes in the run log, and the operator looks at the timer and Cost Explorer every day the host exists. The 32 vCPU quota still caps how much can run at once, and the budget alerts and the budget action are unchanged. The page also says that stopping a host does not save money, because the local disk is lost on stop and a shutdown terminates the instance. It changes no stack, policy, or first-boot code, opens no port, and adds no permission.

How I checked it

scripts/check-repo.py and tofu fmt -check -recursive pass locally, and CI runs validate. The specification of m6id.8xlarge (32 vCPUs, 128 GB, one 1,900 GB local NVMe drive) and the claim that it costs roughly twice the 16 vCPU host are from AWS's published information as I know it. Please confirm both in the console and the pricing calculator before anyone relies on them.

Left out on purpose

• An upper limit on max_lifetime_hours in the variable validation, such as 14 days, so a typo cannot create a months-long host. It changes stack code, so it should be reviewed on its own.

Checklist

• tofu fmt -recursive and tofu validate pass in each stack I changed, or I am relying on CI. No stack changed.
• No state files, .tfvars files, keys, tokens, account IDs, or personal addresses are committed.
• Docs match the change, and follow the house style (US English, no em or en dashes).
• Commits are signed off (DCO). If I forgot, I will post the retroactive sign-off comment described in CONTRIBUTING.md.
• I license this contribution under Apache 2.0.

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