parity: triage 3 false-positives from first-rig run (2026-04-30)

After running the matrix end-to-end against the live rig for the
first time, three of the nine failures were false positives — bugs in
the harness and test invariants, not real backend deltas:

1. ParityHarness configured the legacy backend with
   OTOPCUA_GALAXY_BACKEND=db, which is Discover-only. Reads, writes,
   and reinits all returned "MXAccess code lift pending — DB-backed
   backend covers Discover only". Switched to mxaccess backend; the
   ZB connection string still drives the discovery path.

2. HistoryReadParityTests asserted "neither backend implements
   IHistoryProvider" — but the legacy GalaxyProxyDriver still does
   (it's an accepted back-compat delta retired in PR 7.2). The
   architectural pin we *want* is "the new path doesn't regress to
   per-driver history", so the test now asserts only the mxgw side.

3. AlarmTransitionParityTests strict-pinned the five sub-attribute
   refs (InAlarmRef, etc.) on the legacy condition. PR 2.1 added
   those refs specifically so the new mxgw driver could populate them
   via AlarmRefBuilder; legacy pre-dates PR 2.1 and leaves them null
   — that's correct, not a regression. Test now asserts a one-way
   invariant: when legacy populated a ref, mxgw must match. When
   legacy is null, mxgw is free to populate (the mxgw → server-side
   AlarmConditionService direction).

The six remaining failures are real:

- 2 from the gw-side `[]` array suffix (filed in
  mxaccessgw/requirements-array-suffix-fix.md)
- 2 write-StatusCode mapping deltas (0x80050000 vs 0x80020000) —
  Bad-status both ways but mapped to different OPC UA codes
- 1 event-rate ratio of 5x (mxgw dispatches 5x legacy in the same
  3s window)
- (Plus the 2 ScanState scenarios that skip cleanly — single-platform
  rig as documented)

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

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.ParityTests/ParityHarness.cs

@@ -163,7 +163,12 @@ public sealed class ParityHarness : IAsyncLifetime
                 ["OTOPCUA_GALAXY_PIPE"] = pipe,
                 ["OTOPCUA_ALLOWED_SID"] = sid,
                 ["OTOPCUA_GALAXY_SECRET"] = LegacySecret,
-                ["OTOPCUA_GALAXY_BACKEND"] = "db",
+                // PR 5.W triage 2026-04-30: db-backend is Discover-only. The parity
+                // matrix needs Read / Write / Subscribe over a real MxAccess session,
+                // so use the mxaccess backend. ZB conn string is still consulted for
+                // the discovery path (the mxaccess backend layers MxAccess on top of
+                // the same DB).
+                ["OTOPCUA_GALAXY_BACKEND"] = "mxaccess",
                 ["OTOPCUA_GALAXY_ZB_CONN"] = "Server=localhost;Database=ZB;Integrated Security=True;TrustServerCertificate=True;Encrypt=False;",
             },
         };