v1.0.9-stable — security + correctness

No consensus or wire-format changes vs v1.0.8. Security (HTTP-401 RPC
auth, config double-bind), correctness (CChain TSan-clean), fork
diagnostics (getforkstatus + branchlen + divergence warning), DoS
defence (nMinimumChainWork). Full notes: docs/user/CHANGELOG.md