Minecraft-router chart: naming scheme, hardening, probes, values interface

Stage 2 of the Fleet migration for minecraft-router: bring the chart up to the
repo style guidelines. Chart 0.1.1+up1.25.1 -> 1.0.0+up1.25.1 (major: resource
renames, mappings becomes required, probes and security hardening change the
runtime behavior; appVersion unchanged).

- Resources renamed to {{ .Release.Name }}-{{ .Chart.Name }}-<kind>, templates
  to <chart>.<kind>.yaml; the deployment label now matches its own selector.
- mappings is required and must not be empty (the old default rendered
  "<nil>": "<nil>:25565"); every entry needs domain and internalUrl.
- Ports come from values: server.port (router listen port, also passed as
  --port), server.backendPort (default backend port, overridable per mapping),
  server.apiPort (--api-binding, used by the probes). Defaults stay 25565 /
  25565 / 8080, server.outwardPort keeps its 25565 default and comment.
- _helpers.tpl added (resources/limitFactor and replicas/scaleDown helpers),
  resources come from values, no CPU limit.
- replicas value plus the chart-global scaleDown flag.
- Additive env block rendered through tpl (value / secretKeyRef /
  configMapKeyRef).
- Values-configurable securityContext.pod / .container with hardened defaults:
  non-root uid/gid 10000, readOnlyRootFilesystem, no privilege escalation, all
  capabilities dropped.
- Liveness, readiness and startup probes on the mc-router API: GET /routes with
  an "Accept: application/json" header (the API router matches on that header
  and answers 404 without it).
- icon added to Chart.yaml.
- CI fixture supplies mappings; README chart table entry updated.

ci/minecraft-router/test-values.yaml

@@ -5,9 +5,10 @@
 #    default node-port range (30000-32767). The real cluster runs an extended
 #    --service-node-port-range, the k3d CI cluster does not, so the API server
 #    would reject the Service. This is a CI-only override; the chart default
-#    stays 25565 (see minecraft-router/values.yaml).
-# 2. mappings: the chart default renders "<nil>": "<nil>:25565"; the install
-#    works either way, but the CI install uses a realistic routing table.
+#    stays 25565 (see minecraft-router/values.yaml). server.port (the port the
+#    router itself listens on) keeps its default and is unaffected.
+# 2. mappings: required value without a default - a router without routes is
+#    not operable, so the CI install has to supply a routing table.
 server:
   outwardPort: 30565
 mappings: