Auto: ablegacy-10 — diagnostic counters as tags

Closes #253

docs/drivers/AbLegacy-Test-Fixture.md

@@ -49,6 +49,16 @@ supplies a `FakeAbLegacyTag`.
 - `AbLegacyHostAndStatusTests` — probe + host-status transitions driven by
   fake-returned statuses
 - `AbLegacyDriverTests` — `IDriver` lifecycle
+- `AbLegacyDiagnosticsTests` — PR ablegacy-10 / #253 per-device diagnostic
+  counters: 5 reads (3 ok / 2 fail) → `RequestCount=5`, `ResponseCount=3`,
+  `ErrorCount=2`; `LastErrorCode` reflects the most recent libplctag status;
+  `RetryCount` increments per retry attempt beyond the first; counters reset
+  on `ReinitializeAsync`; discovery emits exactly 7 diagnostic variables per
+  device under `_Diagnostics/`; collision rejection at `InitializeAsync` for
+  user tags shadowing reserved names or `_Diagnostics/` addresses; the
+  `_Diagnostics/<host>/<name>` short-circuit returns the live snapshot through
+  `ReadAsync` without bumping `RequestCount`; two devices keep counters
+  independent.
 - `AbLegacyDeadbandTests` — PR 8 per-tag deadband / change filter:
   absolute-only suppression sequence `[10.0, 10.5, 11.5, 11.6] -> [10.0, 11.5]`,
   percent-only suppression with a zero-prev short-circuit, both-set logical-OR