docs(phase4): Modbus Int64/UInt64, FOCAS fail-fast+scaling, Historian Total+dead-letter cap

docs/AlarmHistorian.md

@@ -111,7 +111,12 @@ collision waits out the file lock instead of failing fast.
 4. The remaining batch is handed to `IAlarmHistorianWriter.WriteBatchAsync`, and
    each outcome is applied in one transaction: `Ack` deletes the row,
    `PermanentFail` flips its `DeadLettered` flag, `RetryPlease` bumps its attempt
-   count and leaves it queued.
+   count and leaves it queued. A row whose `AttemptCount` has reached the configured
+   **`MaxAttempts`** cap (default 10) is dead-lettered automatically on the next drain
+   tick rather than retried — this breaks infinite retry loops for poison events whose
+   payload the historian will always reject (e.g. a malformed alarm record that triggers
+   a permanent SDK error on every attempt). The dead-lettered row remains inspectable
+   via `RetryDeadLettered()` for the configured retention window.
 5. The timer re-arms its next due-time to `max(tickInterval, currentBackoff)`.
 
 **Backoff ladder** (applied to the timer's next due-time, so a historian outage
@@ -196,6 +201,7 @@ When `Enabled` is `false` (the default), `AddAlarmHistorian` registers
 | `DatabasePath` | string | — | Absolute path to the SQLite queue file. Created on first use (WAL mode). Required when `Enabled`. |
 | `SharedSecret` | string | — | Shared secret token the sidecar expects on every connection. Required when `Enabled`. |
 | `BatchSize` | int | `100` | Max rows per drain cycle handed to `IAlarmHistorianWriter.WriteBatchAsync`. |
+| `MaxAttempts` | int | `10` | Maximum delivery attempts before a poison (perpetually-retrying) row is dead-lettered automatically. Must be > 0. |
 | `AlarmHistorian:Host` | string | `localhost` | DNS name or IP of the machine running the historian sidecar. |
 | `AlarmHistorian:Port` | int | `32569` | TCP port the sidecar listens on (`OTOPCUA_HISTORIAN_TCP_PORT`). |
 | `AlarmHistorian:UseTls` | bool | `false` | Wrap the TCP stream in TLS before the Hello handshake. |