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>

apps/longhorn/values.yaml

@@ -0,0 +1,22 @@
+# Values for the upstream Longhorn chart, nested under `longhorn:` (the
+# dependency's name in Chart.yaml).
+longhorn:
+  defaultSettings:
+    # Must match the mount path from the UserVolumeConfig in
+    # talos/patch-longhorn.yaml (/var/mnt/<name>) — this is what points
+    # Longhorn at the dedicated data disk instead of its own default path
+    # on the root filesystem, which on Talos is ephemeral/unsuited for
+    # real data anyway.
+    defaultDataPath: /var/mnt/longhorn
+    # Longhorn's own headroom settings, kept conservative: stop scheduling
+    # new replicas onto a disk once free space drops below these
+    # thresholds, so the cluster degrades gracefully instead of actually
+    # filling a disk.
+    storageMinimalAvailablePercentage: 25
+    storageReservedPercentageForDefaultDisk: 30
+  preUpgradeChecker:
+    # Longhorn's own pre-upgrade check runs as a Job that expects to reach
+    # an upgrade-responder service on the internet by default. Harmless to
+    # leave on, but pointless for a learning cluster and one less thing to
+    # explain — off by default here.
+    jobEnabled: false