Materialize gateway mediation for private resources

[vitramir]

A private resource named by an egress rule moves onto the egress gateway; this service owns every write that makes that true.

• mediation column on private_resources, derived from whether any rule names the resource. SetPrivateResourceMediation (internal, called by the EgressRules service on a resource's first rule and last) rebinds the front service between network-resources-<network> and egress-services, swaps its host.v1 to the forwarding shape, and maintains one private-<id>-upstream-<intercept_port> service per intercept port — each naming its paired target port statically, so choosing the service is what applies the port mapping — plus a single gateway dial policy selecting the shared private-resource-upstreams-<id> attribute.
• Reconciliation re-derives desired mediation per organization from EgressRules.ListMediatedPrivateResources, repairs half-applied flips, and protects in-flight upstream objects from the orphan sweeps by name.
• DeletePrivateResource and a protocol change to tcp are refused while rules name the resource, listing them.
• CreatePrivateResourceAccess for agent and environment principals fast-fails when a public rule already intercepts the resource's hostname for that principal (best-effort, expanded through the agent's environment; EgressRules.ListAttachedRuleDomains).
• ListPrivateResourcesReachableBy (internal) expands grants through the principal's groups (ListMemberGroupsBatch) and, for an agent, its environment, and reports the grants behind each resource for collision reports.
• private_resource.updated is additionally published to the flat private_resources room for the egress gateway's cache invalidation.
• GetPrivateResource treats a caller without identity metadata as mesh-internal (the EgressRules service validating and denormalizing rule targets); identified callers keep the membership check.

Part of egress rules address private resources; protocol change in agynio/api#193.