docs(k8s): add Talos architecture, distro decision, and bootstrap plan

Author the docs/k8s/ directory capturing the pfv-k8s control-plane design:

- README.md: TL;DR of all decisions (distro, runtime, cnode count, admin
  access, identity, tenancy, registry, storage)
- DISTRO-DECISION.md: Talos vs k3s analysis. Recommend Talos because the
  ITAR/classified requirement makes its immutable, API-only, measured-boot
  posture structurally easier to certify than SCAP-hardened Debian. k3s was
  only ever a plan (no cluster deployed), so cutover cost is ~zero.
- ARCHITECTURE.md: target arch with mermaid diagrams covering control
  plane, LAN-only network with Tailscale subnet-router admin, Cilium CNI,
  OIDC to Keycloak, per-tenant vcluster isolation (incl. ITAR tenant),
  Harbor pull-through cache on D3 SSD, bootstrap sequence, and DR.

Gardening: register docs/k8s/ in docmap.md, update STATUS.md with the new
k8s section and the three pending user decisions (cnode count, host spread,
Tailscale pattern).

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docs/k8s/README.md

@@ -0,0 +1,44 @@
+# Kubernetes Architecture & Build Plan
+
+> **Status:** Draft for review. Companion to [`../proxmox/K8S.md`](../proxmox/K8S.md)
+> (which captured the storage/host analysis from the Proxmox audit).
+> This directory takes the next step: **which distro, how to build it,
+> how to operate it.**
+
+**Last updated:** 2026-07-28
+
+## Documents in this directory
+
+| Document | Purpose |
+|----------|---------|
+| [`DISTRO-DECISION.md`](DISTRO-DECISION.md) | Talos vs k3s analysis. Recommendation: **Talos**, with rationale grounded in the ITAR/classified requirement. |
+| [`ARCHITECTURE.md`](ARCHITECTURE.md) | Target architecture: control plane, network, identity, storage, tenant isolation. Mermaid diagrams included. |
+
+## TL;DR
+
+| Decision | Recommendation | Why |
+|----------|----------------|-----|
+| **Distro** | **Talos Linux** (not k3s) | Immutable, API-only, measured-boot-capable. Intrinsically close to a STIG/CIS posture, where k3s-on-Debian requires constant re-certification. The k3s plan was reasonable before the ITAR/classified requirement was on the table. |
+| **Runtime** | **containerd** (Talos default) | Only runtime Talos supports; also the secure default. Non-decision. |
+| **Cnode count** | **3** (down from 5) | Standard HA. Tolerates 1 failure. Frees 2 VMs for tenant worker capacity. Re-evaluate if a regulator mandates 5. |
+| **Admin access** | **Tailscale subnet router** on existing `tailscale-router` VM | Cluster nodes stay LAN-only (no internet egress = stronger ITAR posture). Operator reaches Talos API from anywhere via Tailscale. |
+| **Cluster network** | LAN mTLS only | All cnode/wnode traffic on Proxmox LAN. No Tailscale on cluster nodes themselves. |
+| **Identity** | **OIDC to Keycloak** on Cloudron (production) | Already planned. Talos API + Kubernetes API both trust Keycloak tokens. |
+| **Multi-tenancy** | **vcluster** (per tenant) inside Talos host cluster | Strong workload isolation. One vcluster per tenant: ITAR, non-ITAR, SLP, RackRental. |
+| **Local registry** | **Harbor on D3 SSD** (tsys5, 445 GB free) | Avoids needless round-trips to Docker Hub / Cloudron. Adds image-signing (Cosign) capability for supply-chain integrity. |
+| **Storage classes** | `local-fast`, `nfs-hdd`, `nfs-ssd` | Per [`../proxmox/K8S.md`](../proxmox/K8S.md) §6. Unchanged. |
+
+## What this directory does NOT cover (deferred)
+
+- ETL tooling choice (GDAL/PostGIS/xarray/Dask) — affects RWX vs RWO design.
+- HPC job scheduler (Jobs / Argo Workflows / Volcano) — affects taint/label strategy.
+- vcluster per-tenant policy templates.
+- Solar-aware scale-out (PowerEdge 19xx/2950 hosts) — capacity planning only.
+
+These are tracked as future session work in [`../../STATUS.md`](../../STATUS.md).
+
+## Open question for the user
+
+1. **Cnode count: confirm 3 vs 5.** Recommendation is 3 (rationale in
+   [`ARCHITECTURE.md`](ARCHITECTURE.md) §2). If your ITAR counsel requires
+   2-failure tolerance on the control plane, keep 5.