Move to a three-node cluster with a Talos VIP; add Longhorn

Every node is now both control plane and worker, giving real tolerance
for losing any single machine (etcd quorum needs 3, not 1). A Talos VIP
gives the cluster one stable address regardless of which node answers.

Also pre-installs Longhorn for replicated storage, backed by a Talos
UserVolume on each node's second disk (>=900GB, non-system) rather than
the default path — mirroring the pattern from the homescale repo.

Docs renumbered to make room for 06-longhorn.md; checklist, glossary,
network map, and CLAUDE.md's guided-bootstrap instructions updated to
match, including asking the user for node IPs, VIP, and disk info before
generating any Talos config.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/01-prerequisites.md

@@ -5,31 +5,62 @@ PC, or an amd64 VM). If your hardware is arm64 (Raspberry Pi, Apple Silicon
 VM), the tools below all publish arm64 builds too; swap the architecture in
 whichever install command you use and the rest of this guide is unchanged.
 
-## 1. A target machine
+## 1. Three target machines
 
-You need one machine to become your cluster node — a spare physical PC, a
-mini PC, or a VM (VirtualBox, UTM, Proxmox, or any hypervisor that can boot
-from an ISO). Minimum realistic spec: 2 CPU cores, 4GB RAM, 20GB disk.
+This repo builds a **three-node cluster**, with every node acting as both
+control plane (the brain: API server, scheduler, the cluster's own
+database) and worker (runs your actual apps) at once. Three, not one,
+because it's the smallest number where losing any single machine doesn't
+take the whole cluster down — see
+[`docs/00-what-is-all-this.md`](00-what-is-all-this.md) for why three
+specifically, not two or four.
 
-It needs to be reachable over the network from your everyday machine (the
-one you'll run `talosctl` and `kubectl` from), and it needs a stable IP
-address — see the note about DHCP reservations in
-[`talos/README.md`](../talos/README.md).
+You need three machines — spare physical PCs, mini PCs, or VMs (VirtualBox,
+UTM, Proxmox, or any hypervisor that can boot from an ISO). Minimum
+realistic spec per machine: 2 CPU cores, 4GB RAM, plus **two disks each**:
+one for Talos itself (20GB+ is plenty), and a second, separate disk of
+**at least 900GB** that will be handed entirely to Longhorn
+(see [`docs/06-longhorn.md`](06-longhorn.md)) for storage. If your spare
+hardware doesn't have a second disk that large, that's a real constraint —
+ask Claude about adjusting the size threshold in
+[`talos/patch-longhorn.yaml`](../talos/patch-longhorn.yaml) to fit what you
+actually have.
 
-You do **not** need to install an OS on it yet. That's what the next doc
-covers — Talos gets installed by booting a special ISO and sending it
-configuration over the network, not through a normal installer.
-
-Download the Talos amd64 ISO from the
-[Talos releases page](https://github.com/siderolabs/talos/releases) (look
-for `metal-amd64.iso` under the latest release) and boot your target machine
-from it. Leave it sitting at the boot screen — you'll configure it in
+Each machine needs to be reachable over the network from your everyday
+machine (the one you'll run `talosctl` and `kubectl` from), and each needs
+its own stable IP address — see the note about DHCP reservations in
+[`talos/README.md`](../talos/README.md). You'll also need one *additional*
+unused address for the cluster's shared VIP (Virtual IP) — more on that in
 [`docs/02-talos-bootstrap.md`](02-talos-bootstrap.md).
 
-> **Ask Claude:** if you're not sure whether to use a VM or spare physical
+You do **not** need to install an OS on any of them yet. That's what the
+next doc covers — Talos gets installed by booting a special ISO and
+sending it configuration over the network, not through a normal installer.
+
+### Building your Talos image
+
+Stock Talos doesn't include everything Longhorn needs — specifically, the
+`iscsi-tools` and `util-linux-tools` **system extensions** (optional
+add-ons baked into the boot image itself, since Talos has no package
+manager to install them onto a running node). You build a custom image,
+called a **schematic**, once, and use it for all three nodes:
+
+1. Go to [factory.talos.dev](https://factory.talos.dev).
+2. Under system extensions, select `siderolabs/iscsi-tools` and
+   `siderolabs/util-linux-tools`.
+3. Pick the current stable Talos version and `metal-amd64` as the platform.
+4. Download the resulting ISO (the site calls this a "schematic" —
+   your own custom Talos image, identified by a schematic ID).
+
+Boot **all three** machines from this same ISO, and leave each sitting at
+its boot screen — you'll configure them in
+[`docs/02-talos-bootstrap.md`](02-talos-bootstrap.md).
+
+> **Ask Claude:** if you're not sure whether to use VMs or spare physical
 > hardware, ask — the tradeoffs (snapshots and easy resets vs. more
-> realistic networking) are worth five minutes of discussion before you
-> commit.
+> realistic networking, and whether three VMs on one physical host really
+> gives you the resilience three nodes are supposed to buy you) are worth
+> five minutes of discussion before you commit.
 
 ## 2. talosctl
 
@@ -101,8 +132,12 @@ that references a repo URL means your fork's URL.
 
 ## Checklist
 
-- [ ] Target machine chosen, network-reachable, IP address noted
-- [ ] Talos ISO downloaded, machine booted from it and idle at the boot screen
+- [ ] Three target machines chosen, each network-reachable, each with a
+      stable IP address noted, each with a second disk ≥900GB for Longhorn
+- [ ] One additional unused IP address set aside for the cluster's VIP
+- [ ] Custom Talos schematic built at factory.talos.dev with `iscsi-tools`
+      and `util-linux-tools`, ISO downloaded
+- [ ] All three machines booted from that ISO, idle at the boot screen
 - [ ] `talosctl version --client` works
 - [ ] `kubectl version --client` works
 - [ ] `argocd version --client` works