Resolve all 33 medium-severity architecture review findings

Second slice of the 2026-09-09 whole-system review remediation
(docs/plans/2026-09-09-arch-review.md §10), on top of the High slice 146a1323.

Runtime/devices: SR-2 alarm on-trigger integration scope; SR-3 correlated NativeAlarmActor
subscribe retry; SR-4 immutable audited DB target; SR-5 ordered InstanceActor stop/flush;
SR-12 rehydration fence; SR-13 S&F gate recheck per delivery; DC-4 no OPC UA security
downgrade; DC-5 alarm snapshot for every new subscriber; ORCH-1 sync-dispose joins the pump.

Management/UI: MG-2 site scope on audit tree + Flow Debugger Replay; MG-3 stale-probe memo
removed; MG-4 server-owned bulk-deploy deadline; UI-5 sandbox runs in a killable child
process (new CentralUI.SandboxWorker project); UI-14 bounded sandbox output/IPC.

Edge/audit: EI-1 relay audit ownership; EI-2 SkipBodyCapture suppresses error text; EI-3/EI-11
error-field redaction + caps; EI-4 single relay deadline, pre-execution-only retry; EI-5
authenticated relay provenance; EI-10 body cap before audit read.

Persistence/contracts: DL-1 global Instance.UniqueName unique index + conflict translation;
DL-2 DataConnection protocol config encrypted at rest (migration
20260909203046_EnforceGlobalInstanceNamesAndProtectProtocolConfiguration — coordinated
central upgrade, old binaries cannot read protected rows); DL-3 bounded Replay paging;
DL-6 cross-site Overwrite of a deployed instance rejected; XC-4 proto descriptor contract test.

Config/hygiene: XC-1 central listener isolation by local port; XC-2/XC-3 inert SMTP + cron
keys removed from rig settings; CM-2 env2 bootstrap guard enabled; DC-6 deploy site exit code;
DC-7 notifier retries only explicit standby rejection; XC-5 ClusterClient purged from docs.

Gates recorded in the report: build 0/0, 9 397 passed / 0 failed, rig redeployed on all
eight nodes with the migration applied. 46 Low findings remain open.

docs/requirements/Component-Communication.md

@@ -482,3 +482,20 @@ Site-to-central notification submission and audit ingestion can retry because th
 identifiers are idempotent; status reads and sequence-guarded health reports can retry as well.
 Reconciliation mints fetch tokens and only retries a proven pre-dispatch rejection. Attempts
 share the original absolute deadline. No transport retries `DeadlineExceeded` across nodes.
+
+
+### Source and generated wire contract guard
+
+ProtoDescriptorContractTests invokes the pinned Grpc.Tools protoc for the test host and compares
+the full compiled FileDescriptorProto against each vendored descriptor. This covers message/field
+names, numbers, types, maps, oneofs, enums, options and RPC service signatures, including streaming.
+Mutation controls prove name/type/service drift is detected. Updating a proto requires regenerating
+its vendored C# with docker/regen-proto.sh; changing a source hash alone cannot satisfy the guard.
+The guard requires the package's native compiler to execute on the test host; it does not run
+inside the Docker image build, which retains vendored sources for the documented ARM workaround.
+
+Central listener isolation runs after endpoint routing and before authentication/dispatch. Only
+mapped gRPC methods can execute on the control-plane socket; they are rejected on HTTP/metrics
+sockets. Dedicated metrics sockets expose metrics and health only. Decisions use LocalPort and
+endpoint metadata, never the caller's Host/authority header. The env2 central/site pairs now enable
+the same deterministic bootstrap guard as the main rig for simultaneous container recreation.