test(abcip): hw5 review fixes — HardwareGate guard, (host, tag) allow-list, differential-run refusal (#592)

- HardwareGateGuardTests: only StringWriteGateTests may name a write path in HardwareGate/; the
  allow-list is exactly the two Z-35162 (10.209.70.160) STRING tags; the opt-in, the (host, tag)
  check and the AB_DIFF_ENDPOINT refusal are present. Proven red by a WriteAsync mutation.
- StringWriteGateTests: allow-list entries are (host, tag); any other endpoint is refused; skips
  while AB_DIFF_ENDPOINT is set, like IntegrationWriteGate.
- AbCip.md #566: the close happened somewhere in the 16-60 s window; RST vs FIN not captured.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

docs/drivers/AbCip.md

@@ -180,7 +180,9 @@ The driver has two interchangeable wire transports, selected by one `DriverConfi
   `BadCommunicationError` (transport `BadConnection`); a keep-alive that gets it recycles the connection
   before any request fails. Every other status — tag-level errors, other `0x01` extended codes — is left to
   the request. **Observed on a real CompactLogix (Z-35162, hw5, 2026-10-09): neither code.** A connection left
-  idle past its 16 s timeout (keep-alive disabled for the test) was closed at the TCP level; the next request
+  idle 60 s, past its 16 s timeout (keep-alive disabled for the test), was closed at the TCP level somewhere in
+  that 16–60 s window — the exact moment, and whether the controller sent RST or FIN, were not captured (no
+  packet capture; EPIPE on the first write only says the close was complete by then); the next request
   failed on its write in 9 ms (`Broken pipe` → `BadConnection` → `BadCommunicationError`, no CIP or
   encapsulation status) and the request after it reopened with a fresh Forward Open. That path needs no CIP
   check — a faulted session always reopens on the next send — and is pinned offline by