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-DataConnectionLayer.md

@@ -319,8 +319,8 @@ The registered `SourceReference` is a **NodeId** for OPC UA (the picker, CSV and
 - **Capability check**: if `_adapter is not IAlarmSubscribableConnection`, the actor replies `SubscribeAlarmsResponse(Success = false, ...)`.
 - **Reconnect handling**: on entering **Reconnecting**, the actor pushes a `NativeAlarmSourceUnavailable` to every alarm subscriber (consumers mark mirrored alarms uncertain rather than clearing them). On successful reconnection it re-subscribes the feed; the adapter re-emits a snapshot, reconciling state.
 - **Shared condition filter (last-subscriber-wins)**: because the feed is opened once per source, it carries a single condition filter. A second subscriber that registers a *different* filter for the same source overwrites it (last writer wins) and the actor logs a warning — co-subscribers to one source are expected to agree on the filter.
-- **Routing index**: subscribed sources are bucketed by the **first path segment** of their reference (everything before the first `.`), and a transition is matched only against the bucket for its own first segment, plus a residue list of sources that carry no separator at all (a prefix shorter than one segment, which can match other buckets). This is sound because a source reference containing a separator can only prefix a reference sharing its entire first segment. The per-source `StartsWith` test and the condition-type gate inside the bucket are unchanged, so routing decisions are identical to the previous linear scan over every subscribed source; snapshot boundaries bypass the condition filter, but OPC UA boundaries are sent only to the exact source binding. Only legacy connection-wide Mx boundaries broadcast to all subscribers.
-- **Gateway union filter (MxGateway)**: `StreamAlarms` carries a single `alarm_filter_prefix`, so the adapter opens the stream on the **longest common prefix** of the currently subscribed source references and restarts it only when a NEW source falls outside that prefix (the source then replays a fresh snapshot, which `NativeAlarmActor` already handles). An unsubscribe never restarts the stream — the prefix it leaves behind is at worst too broad, which costs bandwidth, not correctness. The prefix is a bandwidth optimisation only; the actor's per-source + condition-type gate remains authoritative, mirroring the OPC UA server-side WhereClause stance.
+- **Routing index**: subscribed sources are bucketed by the **first path segment** of their reference (everything before the first `.`), and a transition is matched only against the bucket for its own first segment, plus a residue list of sources that carry no separator at all (a prefix shorter than one segment, which can match other buckets). This is sound because a source reference containing a separator can only prefix a reference sharing its entire first segment. The per-source `StartsWith` test and the condition-type gate inside the bucket are unchanged, so routing decisions are identical to the previous linear scan over every subscribed source; snapshot boundaries bypass the condition filter, but OPC UA boundaries are sent only to the exact source binding. Connection-wide Mx boundaries broadcast to all subscribers while preserving their refresh epoch.
+- **Gateway union filter (MxGateway)**: `StreamAlarms` carries a single `alarm_filter_prefix`, so the adapter opens the stream on the **longest common prefix** of the currently subscribed source references and restarts it for each new source, including one covered by the existing prefix, so every newly bound source receives standing state. Each transport refresh has connection-wide start/end markers and a distinct epoch. An unsubscribe never restarts the stream — the prefix it leaves behind is at worst too broad, which costs bandwidth, not correctness. The prefix is a bandwidth optimisation only; the actor's per-source + condition-type gate remains authoritative, mirroring the OPC UA server-side WhereClause stance.
 
 ### Protocol-Neutral Types & Messages
 
@@ -459,3 +459,10 @@ The DCL reports the following metrics to the Health Monitoring component via the
 - **Site Runtime (NativeAlarmActor)**: Bidirectional — receives alarm subscribe/unsubscribe requests, delivers native alarm transitions and source-unavailable notifications (read-only; no ack-back to the source).
 - **Health Monitoring**: Reports connection health periodically.
 - **Site Event Logging**: Logs connection/disconnection events.
+
+
+### Medium architecture review connection guarantees (2026-09-09)
+
+OPC UA connection and endpoint verification select exactly the configured security mode. If a protected mode is unavailable, or discovery fails while protection was requested, the operation fails with the requested mode in its diagnostic; it never falls back to an unsecured session.
+
+DCL retains each subscribed alarm source's current standing state and its in-progress snapshot. A new subscriber sharing a feed receives a scoped replay, or the already-received prefix of an in-progress frame followed by the remaining frame. Cache state is discarded on reconnect and last-owner unsubscribe. A new Mx source reopens the shared stream even when the existing prefix covers it, obtaining a fresh standing-state snapshot; callbacks from the canceled stream are ignored. Mx refresh boundaries remain connection-wide but share a distinct per-stream epoch. Both replay cache and runtime mirror ignore stale-epoch rows and apply newer live raises/clears over a concurrent refresh before committing it.