feat: add Gateway API HTTPRoute support with optional self-managed Gateway

[disco-stu]

Summary

Adds opt-in Gateway API routing as an alternative to Ingress. Every BaSyx
service can now be exposed via a Gateway API HTTPRoute instead of (or
alongside) an Ingress, and the chart can optionally manage its own
Gateway resource for standalone/demo deployments that don't already have
one. Ingress remains the default routing mechanism; the feature is disabled
by default and fully backwards compatible.

Changes

• Add <service>.httpRoute.{enabled,parentRefs,annotations,hosts} values and
  a templates/<service>/httproute.yaml for all 11 BaSyx services
• Add a common.httpRoute.parentRefs helper resolving parentRefs with this
  precedence: service-local > chart-wide gatewayApi.parentRefs >
  auto-derived reference to the chart-managed Gateway > explicit fail
  with an actionable error message
• Add an optional self-managed Gateway resource
  (gatewayApi.gateway.enabled, default false) with fixed http/https
  listeners, standard chart labels, and className validation via
  required(...)
• Derive cert-manager.io/issuer / cert-manager.io/cluster-issuer
  annotations on the Gateway from gatewayApi.gateway.issuer /
  clusterIssuer, deliberately decoupled from the existing ingress.issuer
  / ingress.clusterIssuer values
• Extend values.schema.json with the new gatewayApi object
• Add two example values files: values/values.gatewayapi.example.yaml
  (bring-your-own Gateway) and values/values.gatewayapi-managed.example.yaml
  (chart-managed Gateway)
• Document the feature in README.md, including a new "Gateway API
  (HTTPRoute)" section with value tables and the parentRefs precedence
  rules
• Bump chart version to 3.12.0 (appVersion unchanged at 1.0.11, no
  BaSyx Go image change)

Validation

• helm lint charts/basyx – 0 errors
• helm unittest charts/basyx – 18 suites / 242 tests passing, including
  new gateway_test.yaml and httproute_test.yaml
• Manually verified against a real Envoy Gateway installation (Gateway API
  implementation)