feat(stalwart): spread each node group over nodes and give each one a PDB

A two-replica node group was redundancy on paper: nothing kept both pods off
one node, and no PodDisruptionBudget kept a drain from taking both at once.
Both objects are now rendered per node group, gated on that group's COMPUTED
replica count being >= 2.

The distribution is a promise, not an effort: maxSkew 1 on
kubernetes.io/hostname with whenUnsatisfiable DoNotSchedule, selecting only
this group's own pods, so an "mx" and a "full" pod may share a node. The
selector comes from the chart (stalwart.groupLabels), never from the operator -
a spread constraint whose selector matches no pod is trivially satisfied,
distributes nothing and reports nothing.

nodeAffinityPolicy: Ignore is set explicitly, and that is the load-bearing
detail. Its API default is Honor, under which a narrowing nodeSelector shrinks
the set of counted domains until both replicas fit into one: skew 0, both pods
co-located, no Pending, no event, and a selector that is correct - so no check
can see it. With Ignore the same narrowing gives skew 2 and the second pod is
Pending. A render abort was the alternative and would also have forbidden a
nodeSelector spanning five nodes, which is legal.

A user entry colliding with the chart's own on (topologyKey,
whenUnsatisfiable) aborts the render naming both values, because the API
rejects the duplicate with a message about a list index and an apply-based path
merges the two over patchMergeKey: topologyKey silently, dropping one selector.

Minor rather than major, proved with a diff rather than argued: with the
shipped defaults (full: 1, mx: 0) and with ci/stalwart/test-values.yaml,
helm template renders byte-for-byte what 1.0.1+upv0.16.24 rendered - both
diffs empty.

New in CI, over every chart on every PR: ci/run-selector-match-test.sh holds
every rendered PodDisruptionBudget selector and every topologySpreadConstraints
labelSelector against the labels of the pod templates in the same render. Helm
itself is the YAML parser (ci/yaml2json), because the runners have neither
pyyaml nor yq and a hand-written parser could read nothing and report nothing.
podAffinity/podAntiAffinity are out of scope; matchExpressions-only selectors
are reported as not checked rather than counted as passing. Two anti-vacuum
guards: a chart with an overlay under ci/selector-match/ that renders no
selector fails, and a run that checks no selector at all fails.

Fixes #96

stalwart/Chart.yaml

@@ -18,7 +18,14 @@ type: application
 # all three of latest / v0.16 / v0.16.24 resolved to one digest pushed seconds
 # apart, so either floating name would change the running image with no commit
 # in this repository and nothing to report it.
-version: 1.0.1+upv0.16.24
+# MINOR, not major, and the evidence is a diff rather than a judgement: the
+# per-group spread constraint and the per-group PodDisruptionBudget of Gitea #96
+# are both gated on the COMPUTED replica count being >= 2, and the chart ships
+# full: 1 / mx: 0. `helm template` against the previous version therefore renders
+# byte-for-byte identically for the defaults AND for ci/stalwart/test-values.yaml
+# - measured, both empty diffs. New optional values with unchanged defaults is
+# exactly the minor case (README, "Versioning").
+version: 1.1.0+upv0.16.24
 appVersion: "v0.16.24"
 
 maintainers: