feat(control-plane): require complete plan publication

[zzylol]

Summary

• make BackendPlan a required member of each coupled publication generation
• report per-document streaming/routing/plan status on desynchronization
• refuse partial publication when BackendPlan emission fails
• distinguish the uninitialized bootstrap plan from an installed active plan
• keep legacy serving available only before the first real plan activation

Validation

• cargo test -p control_plane emit::backend_push --lib (15 passed)
• cargo test -p data_plane --test e2e_controller_plans_and_backend_serves controller_really_pushes_backend_plan_and_query_really_serves
• git diff --check

Stack

Depends on #465.