Auto: abcip-5.1 — HSBY paired-IP role probing

Closes #242

docs/drivers/AbServer-Test-Fixture.md

@@ -160,6 +160,28 @@ The driver implements all of these + they have unit coverage, but the only
 end-to-end paths `ab_server` validates today are atomic `ReadAsync` and
 write-deadband / write-on-change suppression.
 
+### 8. ControlLogix HSBY paired-IP role probing (PR abcip-5.1)
+
+`ab_server` has no second-chassis concept and no `WallClockTime.SyncStatus`
+tag. The HSBY paired-IP role-prober (PR abcip-5.1) is unit-tested only —
+`AbCipHsbyTests` drives two fake runtimes (primary + partner), pins each
+chassis's role-tag value, and asserts the active-resolution rules + DTO
+round-trip + diagnostics surface.
+
+The `paired` Docker compose profile spins up two `ab_server` instances +
+a stub `hsby-mux` sidecar so the topology is documented, but PR 5.2 follow-
+up needs a patched `ab_server` image (or a Python shim) that actually
+serves the role tag before the integration test
+(`AbCipHsbyRoleProberTests`) can flip its `Assert.Skip` into a real wire
+assertion. Until then the test is gated on `Category=Hsby` + skipped by
+default.
+
+Lab-rig coverage is the authoritative path — a real 1756-RM redundant
+chassis pair is the only place the live `WallClockTime.SyncStatus` matrix
++ split-brain handling can be exercised end-to-end. See
+[`AbCip-HSBY.md`](AbCip-HSBY.md) for the full configuration + role-tag
+detection matrix.
+
 ## Logix Emulate golden-box tier
 
 Rockwell Studio 5000 Logix Emulate sits **above** ab_server in fidelity +