Auto: ablegacy-12 — auto-demote on comm failure

Closes #255

docs/drivers/AbLegacy-Test-Fixture.md

@@ -53,12 +53,31 @@ supplies a `FakeAbLegacyTag`.
   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.
+  on `ReinitializeAsync`; discovery emits the canonical diagnostic variables
+  per device under `_Diagnostics/` (now 9 with PR ablegacy-12); 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.
+- `AbLegacyAutoDemoteTests` — **PR ablegacy-12 / #255** auto-demote on comm
+  failure: 3 consecutive failures arm the demote window and surface
+  `HostState.Demoted`; subsequent reads short-circuit with
+  `BadCommunicationError` *without invoking libplctag* (verified via
+  `factory.Tags["N7:0"].ReadCount` not advancing); successful read resets
+  the consecutive-failure counter; failure-success-failure pattern doesn't
+  cross the threshold; `DemoteCount` + `LastDemotedUtc` surface via
+  `_Diagnostics/`; `Enabled=false` opts out (failures still count, demotion
+  never fires); `ReinitializeAsync` clears the active window but preserves
+  cumulative `DemoteCount`; cool-down expiry allows the next read through;
+  two devices in one driver — one faulty, one healthy — proves the faulty
+  side's demotion doesn't starve the healthy side; `BadNodeIdUnknown`
+  (terminal) does not count toward the comm-failure tally; DTO JSON
+  round-trip preserves `FailureThreshold` / `DemoteForMs` / `Enabled` at
+  the per-device level; `HostState.Demoted` enum value is wired through
+  `Core.Abstractions`. Companion integration test in
+  `tests/.../IntegrationTests/AbLegacyAutoDemoteTests.cs` runs the
+  two-device-one-unreachable scenario against a live ab_server fixture
+  using `127.0.0.1:1` as the unreachable peer.
 - `RsLogixSymbolImportTests` — ablegacy-11 / #254 RSLogix CSV symbol-import parser:
   canonical 8-row CSV (one row per N/F/B/L/ST/T/C/R) → 8 typed
   `AbLegacyTagDefinition`s with the right `DataType`; header + comment-line