fix(historian): #690 Gateway raw/AtTime Bad rows carry no value (a stored NULL was value 0)

docs/Historian.md

@@ -686,7 +686,12 @@ data.
   the gateway is asked from **1 ms before** the window (never before the Unix epoch, the start of the historian's time encoding),
   so the boundary row lands before it, and only rows `RawAggregateReader.IsWindowSample` are returned. The decision is by
   **timestamp**, not by value or quality — a stored NULL sample looks exactly like the no-data boundary row on the wire (value 0,
-  Bad), and a stored NULL inside the window is still returned (Bad), as it is a stored sample. A row stamped **at or before the
+  Bad), and a stored NULL inside the window is still returned (Bad), as it is a stored sample. ⚠️ **Since #690 it is returned
+  with a null value** (client-visible): histsdk puts a value on every row, so the NULL used to come back as value `0` + Bad (`""`
+  on a string tag), a fabricated 0 for any client that reads the value without the status. `SampleMapper.ToSnapshot` drops the
+  value of every row whose mapped status is **Bad** (AVEVA stores a Bad-quality sample as NULL, so a Bad row never carries a real
+  value) — raw reads and AtTime alike, matching SqlServer/QuestDB (an Empty sample reads null + Bad). Uncertain rows keep their
+  value; the raw-evaluated aggregates were already unaffected (a Bad sample is never usable, and Start/End null a Bad value). A row stamped **at or before the
   epoch** is never a stored sample (AVEVA stores nothing there — the #686 walk-back's floor too): a window starting at or before
   the epoch gets no margin, so AVEVA's no-data row lands exactly at its start; raw reads and the per-sample aggregates both drop it.
 - **The forward end is exclusive** (`[StartTime, EndTime)`, the `IHistorianDataSource` contract and what the SQL backends return;