Document the trailing 35 bytes of GetNextQueryResultBuffer rows

Investigation finding only — no behavior change, no new public fields.

Captured a fresh GetNextQueryResultBufferResponse for SysTimeSec via
instrument-wcf-readmessage and compared against the canonical 4-row
OtOpcUaParityTest_001.Counter fixture. Trailing-block structure is
tag-independent:

  bytes 0-2   constant 0x00 0x00 0x01 (sample-format marker)
  bytes 3-10  Int64 FILETIME UTC (duplicate of startTime for raw rows;
              already used by the aggregate parser as the interval start)
  bytes 11-18 zeros (likely end-time slot — populated by aggregate variants)
  bytes 19-26 varies row-to-row even with identical Quality/Value;
              looks like a storage block sequence ID or snapshot offset
  bytes 27,29 flag bytes (0/1 and 0/4 observed); semantics undecoded
  bytes 28, 30-34 zeros

None of bytes 19-34 have a clear user-facing meaning; they appear to be
server-internal storage metadata. Updated the
TryParseGetNextQueryResultBufferRows remarks block with the byte map and
a note that surfacing them as new HistorianSample fields should wait
until a customer actually asks. CLAUDE.md "Remaining gaps" entry updated
to reflect the new partial decode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -95,7 +95,7 @@ Smaller, isolated items — none block the production read surface:
 
 - Remote TCP transports (`RemoteTcpIntegrated`, `RemoteTcpCertificate`) untested against an actual remote Historian (tests skip without `HISTORIAN_REMOTE_TCP_HOST`).
 - Explicit username/password tag-metadata path is wired (validator only blocks no-auth-at-all), but live-verification requires `HISTORIAN_USER`+`HISTORIAN_PASSWORD` set; gated test `GetTagMetadataAsync_ExplicitCredentials_AgainstLocalHistorian` skips otherwise.
-- Per-row trailing ~24 bytes of `GetNextQueryResultBuffer` are not decoded (likely per-sample value/source/state metadata).
+- Per-row trailing 35 bytes of `GetNextQueryResultBuffer` are now mapped (see `HistorianDataQueryProtocol.TryParseGetNextQueryResultBufferRows` doc comment) — bytes 3-10 = duplicate FILETIME (already used by aggregate parser), bytes 0-2 + 19-34 = server-internal sample/storage metadata with no clear user-facing meaning. No new public fields added; revisit if a customer asks for storage metadata exposure.
 - `EnsureTagAsync` distinct `MinRaw`/`MaxRaw` persistence requires `ApplyScaling=true` + a follow-up `UpdateTags` call — not yet wired (no API user has asked).
 
 ### Tools Layer

src/AVEVA.Historian.Client/Wcf/HistorianDataQueryProtocol.cs

@@ -18,10 +18,24 @@ internal static class HistorianDataQueryProtocol
     /// the canonical OtOpcUaParityTest_001.Counter capture, 4 rows × 141 bytes inside a 570-byte body):
     /// header is UInt16 version=9 + UInt32 rowCount; each row is UInt32 tagKey + UInt32 tagNameLen +
     /// (tagNameLen × 2) UTF-16 chars + UInt32 sampleCount + Int64 startUtc FILETIME + UInt32 quality +
-    /// UInt32 qualityDetail + UInt32 opcQuality + Double numericValue + Double percentGood + 1-byte
-    /// marker + 34 trailing bytes whose meaning is undecoded for raw rows. The 5-byte error/terminal
-    /// buffer accompanying the result decodes as `04 1E 00 00 00` = type 4, code 30 = "no more data";
-    /// any other shape leaves <paramref name="hasMoreData"/> true.
+    /// UInt32 qualityDetail + UInt32 opcQuality + Double numericValue + Double percentGood + 35-byte
+    /// trailing block. The 5-byte error/terminal buffer accompanying the result decodes as
+    /// `04 1E 00 00 00` = type 4, code 30 = "no more data"; any other shape leaves
+    /// <paramref name="hasMoreData"/> true.
+    ///
+    /// Trailing 35 bytes (cross-tag verified 2026-05-04 against SysTimeSec — structure is
+    /// tag-independent, server-internal sample metadata):
+    ///   bytes 0-2   constant 0x00 0x00 0x01 (sample-format marker)
+    ///   bytes 3-10  Int64 FILETIME UTC — duplicate of startTime for raw rows;
+    ///               aggregate parser reads it as the interval start (offset row+tail+43)
+    ///   bytes 11-18 zeros (reserved — likely end-time slot, populated by aggregate variants)
+    ///   bytes 19-26 varies row-to-row even for identical Quality/Value; likely a storage
+    ///               block sequence ID or snapshot offset. No user-facing meaning surfaced.
+    ///   bytes 27,29 flag bytes (0/1 and 0/4 observed); semantics undecoded
+    ///   bytes 28, 30-34 zeros (reserved)
+    /// No public HistorianSample fields map to bytes 19-34 — they look like server-internal
+    /// storage metadata. If a customer ever needs them surfaced, capture more rows with
+    /// known-distinct properties (force-store, backfill, version-replace) to narrow down.
     /// </remarks>
     public static bool TryParseGetNextQueryResultBufferRows(
         ReadOnlySpan<byte> result,