Task #253 follow-up — bidirectional + subscribe-sees-change e2e stages

The original three-stage design (probe / driver-loopback / forward-
bridge) only proved driver-write → server-read. It missed:

 - OPC UA write → server → driver → PLC (the reverse direction)
 - server-side data-change notifications actually firing (a stale
   subscription can still let a read-after-the-fact return the new
   value and look fine)

Extend _common.ps1 with two helpers:

 - Test-OpcUaWriteBridge: otopcua-cli write the NodeId -> wait 3s ->
   driver CLI read the PLC side, assert equality.
 - Test-SubscribeSeesChange: Start-Process otopcua-cli subscribe in the
   background with --duration N, settle 2s, driver-side write, wait for
   the subscription window to close, assert captured stdout contains
   the new value.

Wire both into test-modbus / test-abcip / test-ablegacy / test-s7 /
test-focas / test-twincat after the existing forward-bridge stage.
Update README to describe the five-stage design + note that the
published NodeId must be writable for stages 4 + 5.

Also prepend UTF-8 BOM to every script in scripts/e2e so Windows
PowerShell 5.1 parsers agree on em-dash byte sequences the way
PowerShell 7 already does. The scripts still #Requires -Version 7.0 —
the BOM is purely defensive for IDE / CI step parsers.

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

scripts/e2e/test-abcip.ps1

@@ -1,4 +1,4 @@
-#Requires -Version 7.0
+#Requires -Version 7.0
 <#
 .SYNOPSIS
   End-to-end CLI test for the AB CIP driver (ControlLogix / CompactLogix /
@@ -6,7 +6,8 @@
 
 .DESCRIPTION
   Mirrors test-modbus.ps1 but against libplctag's ab_server (or a real Logix
-  controller). Three assertions: probe / driver-loopback / server-bridge.
+  controller). Five assertions: probe / driver-loopback / forward-bridge /
+  reverse-bridge / subscribe-sees-change.
 
   Prereqs:
     - ab_server container up (tests/.../AbCip.IntegrationTests/Docker/docker-compose.yml,
@@ -81,5 +82,23 @@ $results += Test-ServerBridge `
     -OpcUaNodeId $BridgeNodeId `
     -ExpectedValue "$bridgeValue"
 
+$reverseValue = Get-Random -Minimum 20000 -Maximum 29999
+$results += Test-OpcUaWriteBridge `
+    -OpcUaCli $opcUaCli `
+    -OpcUaUrl $OpcUaUrl `
+    -OpcUaNodeId $BridgeNodeId `
+    -DriverCli $abcipCli `
+    -DriverReadArgs (@("read") + $commonAbCip + @("-t", $TagPath, "--type", "DInt")) `
+    -ExpectedValue "$reverseValue"
+
+$subValue = Get-Random -Minimum 30000 -Maximum 39999
+$results += Test-SubscribeSeesChange `
+    -OpcUaCli $opcUaCli `
+    -OpcUaUrl $OpcUaUrl `
+    -OpcUaNodeId $BridgeNodeId `
+    -DriverCli $abcipCli `
+    -DriverWriteArgs (@("write") + $commonAbCip + @("-t", $TagPath, "--type", "DInt", "-v", $subValue)) `
+    -ExpectedValue "$subValue"
+
 Write-Summary -Title "AB CIP e2e" -Results $results
 if ($results | Where-Object { -not $_.Passed }) { exit 1 }