docs(azure): decide Standard + NAP over AKS Automatic

docs/azure/cluster-mode.md

@@ -0,0 +1,76 @@
+# AKS cluster mode: Automatic vs Standard + Node Auto-Provisioning
+
+**AKS Standard + Node Auto-Provisioning.** NAP runs the same engine
+Automatic would have preconfigured; the upgrade channel, node-image
+patching, and policy enforcement stay the factory's to set.
+
+Need Windows node pools or an IPv6 cluster? NAP supports neither, on any
+tier — Socle is not the right foundation for that cluster.
+
+## What this bundles
+
+- NAP provisions and scales workload nodes — the same open-source engine
+  Automatic uses, opt-in and explicit here instead of preconfigured.
+- No separate Azure charge for NAP itself: the retail price list carries a
+  per-category compute meter only under the Automatic product — Standard
+  has none. Node price is identical to running Karpenter self-hosted; NAP's
+  advantage is skipping the operational cost of running that controller,
+  not a lower bill.
+- Upgrade channel, node OS image upgrade, and policy enforcement are the
+  factory's to configure — nothing locked.
+- NAP's own limits apply regardless of tier: no Windows node pools, no IPv6
+  clusters.
+
+## The two options
+
+|  | Standard + NAP | Automatic |
+| --- | --- | --- |
+| Per-vCPU surcharge | None | +17.5% on On-Demand, +94.7% on Spot |
+| Pod readiness SLA | None | 99.9% of qualifying operations in under 5 minutes |
+| Upgrade channel, policy enforcement | Factory's choice | Forced |
+| System components | Factory-operated | Microsoft-operated, no access |
+
+## Cost
+
+- Reference estate: one prod environment 24/7, two UAT environments at 12h
+  per working day. Two nodes per environment (one Standard_D4s_v5, one
+  Standard_D8s_v5 — 12 vCPU / 48 GiB total), one cluster per environment.
+- France Central list price, September 2026.
+
+| Estate of three environments | Standard + NAP | Automatic |
+| --- | --- | --- |
+| Nodes (identical price either way) | $841.34 | $841.34 |
+| Control plane | $219.00 | $350.40 |
+| Per-vCPU surcharge | — | $147.25 |
+| **Total per month** | **$1,060.34** | **$1,338.99** |
+
+The surcharge is a fixed $/vCPU/h, not a percentage of what the node
+actually costs — so it hits hardest exactly where compute is cheapest. On
+On-Demand it's +17.5%. On Spot, the same fixed amount is **+94.7%**,
+because the underlying VM price it's added to is so much lower.
+
+## Why, beyond cost
+
+Automatic's own tax doesn't scale down with Spot — it stays a fixed
+$/vCPU/h, so on Spot it costs nearly as much as the compute itself.
+Spot's own discount is untouched; the tax on top of it isn't. Against
+that, and against a pod-readiness SLA and not having to pick an upgrade
+channel or a policy mode, Automatic isn't worth its permanent cost.
+
+## What you give up
+
+- The pod-readiness SLA (99.9% of qualifying operations in under 5
+  minutes) — Automatic-only, financially backed.
+- A hands-off upgrade channel and policy baseline: the factory now picks
+  and maintains both instead of inheriting Microsoft's default.
+
+## Sources
+
+Read September 2026, France Central pricing.
+
+[AKS Automatic overview][automatic] · [node auto-provisioning][nap] ·
+[Azure Retail Prices API][retail-prices].
+
+[automatic]: https://learn.microsoft.com/en-us/azure/aks/intro-aks-automatic
+[nap]: https://learn.microsoft.com/en-us/azure/aks/node-auto-provisioning
+[retail-prices]: https://learn.microsoft.com/en-us/rest/api/cost-management/retail-prices/azure-retail-prices