feat(runtime): use Core registration leases

[XnLemon]

Summary

• replace the Samples-owned Nacos wire implementation with Core InstanceRegistrar / InstanceLease
• bind Runtime A and Runtime B registration to an exact Agent Card/Release target with explicit freshness values
• fail startup on initial registration failure, terminate readiness/serving on lease failure, and explicitly deregister on shutdown
• add owning-package lifecycle/config tests and include them in CI race/vet coverage

Dependency

Stacked on NeKiro-project/NeKiro#106 at exact Core commit 3e815b89cb3799767632291dc882f20e3a95b61c. No local replace or floating revision is used.

Validation

• go mod tidy and clean module diff
• go build ./...
• go test ./...
• go test -count=1 ./internal/nacosregistration/... ./runtime-b/... ./runtime-a/...
• go test -race ./internal/nacosregistration/... ./runtime-b/... ./runtime-a/...
• go vet ./internal/nacosregistration/... ./runtime-b/... ./runtime-a/...
• direct internal/nacosregistration coverage: 75.8%

Fallback delta: removed 0, retained 0, added 0, net +0.
Added fallback evidence: none.