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

docs/Historian.md

@@ -154,6 +154,23 @@ are disposed when the session closes). Resuming an unknown / evicted / released
 `BadContinuationPointInvalid`. `releaseContinuationPoints` drops the stored cursors without reading
 data.
 
+### Total aggregate derivation
+
+The OPC UA `Total` aggregate is **supported** over the Wonderware backend. Because the
+Wonderware `AnalogSummary` query exposes no `Total` column, the value is derived client-side
+using the time-integral identity:
+
+> **Total = time-weighted Average × interval-seconds**
+
+The wire request is issued with the `Average` column; each returned bucket's value is
+multiplied by `interval.TotalSeconds` before the result is returned to the OPC UA client.
+Bucket status codes and timestamps are preserved unchanged. Null (unavailable) Average
+buckets produce a null Total (`BadNoData` downstream) — the scaling is not applied.
+
+This derivation is exact for piecewise-constant (step) signals. For continuously varying
+signals it is an approximation identical to the one Wonderware would apply internally, so
+the result is consistent with what AVEVA Historian reports for the same window.
+
 ### Known limitations
 
 - **Processed and AtTime are single-shot** (no continuation points). Unlike Raw, neither
@@ -225,9 +242,11 @@ otopcua-cli historyread \
   -U reader -P password
 ```
 
-Supported `--aggregate` values: `Average`, `Minimum`, `Maximum`, `Count`, `Start`, `End`,
-`StandardDeviation` (aliases: `avg`, `min`, `max`, `stddev`/`stdev`, `first`, `last`).
+Supported `--aggregate` values: `Average`, `Minimum`, `Maximum`, `Total`, `Count`, `Start`, `End`,
+`StandardDeviation` (aliases: `avg`, `min`, `max`, `total`, `stddev`/`stdev`, `first`, `last`).
 `--interval` is the processing interval in milliseconds (default 3600000 = 1 hour).
+`Total` is derived client-side as time-weighted Average × interval-seconds (see "Total aggregate
+derivation" above).
 
 ---