feat(historian-gateway): native TimeAverage/Total on by default, SendEvents batches, server-side max_events (#694, #746)

#694 item 3 — historiangw#27 is fixed and now live-verified through OtOpcUa:
- ServerHistorian:Gateway:EnableNativeTimeWeighted defaults to true (false is
  the kill switch for a gateway older than 9f711c1). TimeAverage / Total are
  served and advertised again (12 aggregates).
- The gateway sends the interval as max(1, round(ms)) and AVEVA clips its last
  interval at the request end (live). NativeAggregatePlan therefore sends a
  multi-interval grid only for whole-ms intervals; a fractional interval is read
  one interval per request (at most 1,000, else BadTooManyOperations), and every
  one-interval request goes out rounded UP to whole ms so AVEVA answers one
  clipped row. Live, a 3.0004 s clipped interval sent at its own width lost its
  last 0.4 ms (Total 123 vs 123.0172).

#746 — HistorianGateway Contracts/Client 1.0.0 surface:
- Alarm history: GatewayAlarmHistorianWriter sends SendEvents batches of <=200.
  A failed per-event result is re-sent alone via SendEvent (classified as
  before); a refused batch (InvalidArgument etc.) goes event by event so a
  poison event dead-letters alone; a systemic fault retries the rest without
  sending more; Unimplemented (older gateway) falls back to SendEvent for good.
  IHistorianGatewayClient.SendEventsAsync defaults to Unimplemented.
- Events reads: a capped page sends cap + 1 as ReadEventsRequest.max_events, so
  the gateway completes the stream instead of the client cancelling it. An
  ambiguous capped answer (foreign rows filtered out of the server's cut) or a
  refused cap is re-read uncapped with the existing EventDrainBudget drain.

Tests: GatewayServerCappedEventsTests, GatewayAlarmWriterBatchTests,
GatewayNativeWholeMillisecondGridTests (gateway-faithful rounding fake), gate
tests flipped to the new default; every new test mutation-checked (each
mutation of the production change fails at least one). Live harness
AvevaGatewayHistorianFollowupsLiveGateTests (read-only) for #694/#700/#746.

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

docs/AlarmHistorian.md

@@ -53,7 +53,9 @@ unless noted.
 - **`IAlarmHistorianWriter`** — what the drain worker delegates writes to.
   `WriteBatchAsync(batch, ct)` returns one `HistorianWriteOutcome` per event,
   in order. Production binds this to `GatewayAlarmHistorianWriter` (the
-  HistorianGateway `SendEvent` path).
+  HistorianGateway `SendEvents` batch path — up to 200 events per RPC, #746 —
+  with `SendEvent` as the per-event fallback for a failed event, a refused
+  batch, or a gateway without the batch RPC).
 - **`HistorianWriteOutcome`** — per-event drain result: `Ack` (persisted,
   remove from queue), `RetryPlease` (transient failure — leave queued, retry
   after backoff), `PermanentFail` (malformed/unrecoverable — move to
@@ -241,7 +243,7 @@ writes across a redundant pair. `AlarmTransitionEvent` carries `AlarmTypeName`
 (the Part 9 subtype string) and `Comment` (the operator comment from the
 originating ack/shelve command) that populate the corresponding fields of
 `AlarmHistorianEvent`. `GatewayAlarmHistorianWriter` is the `IAlarmHistorianWriter`
-the drain worker delegates to (the gateway `SendEvent` path). See
+the drain worker delegates to (gateway `SendEvents` batches, `SendEvent` fallback). See
 [ServiceHosting.md](ServiceHosting.md) for the (external) HistorianGateway setup.
 
 **Scope:** scripted alarms only. Galaxy-native alarms historize via System
@@ -285,7 +287,7 @@ node's consolidated `LocalDb:Path` database (see
 
 | Key | Type | Default | Description |
 |---|---|---|---|
-| `Enabled` | bool | `false` | Enable the durable store-and-forward sink (drains to the HistorianGateway `SendEvent` path). `false` → `NullAlarmHistorianSink`. Requires LocalDb to be registered — an enabled sink with no `ILocalDb` fails at resolution rather than silently discarding events. |
+| `Enabled` | bool | `false` | Enable the durable store-and-forward sink (drains to the HistorianGateway `SendEvents` / `SendEvent` path). `false` → `NullAlarmHistorianSink`. Requires LocalDb to be registered — an enabled sink with no `ILocalDb` fails at resolution rather than silently discarding events. |
 | `BatchSize` | int | `100` | Max rows per drain cycle handed to `IAlarmHistorianWriter.WriteBatchAsync`. |
 | `DrainIntervalSeconds` | int | `5` | Seconds between drain-worker ticks. |
 | `Capacity` | long | `1000000` | Max queued rows before the sink evicts the oldest (data-loss signal via `EvictedCount`). |