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/_common.ps1

@@ -1,4 +1,4 @@
-# Shared PowerShell helpers for the OtOpcUa end-to-end CLI test scripts.
+# Shared PowerShell helpers for the OtOpcUa end-to-end CLI test scripts.
 #
 # Every per-protocol script dot-sources this file and calls the Test-* functions
 # below. Keeps the per-script code down to ~50 lines of parameterisation +
@@ -202,6 +202,114 @@ function Test-ServerBridge {
     return @{ Passed = $false; Reason = "bridge value mismatch" }
 }
 
+# Test 4 — reverse bridge. Write via the OPC UA client CLI, then read the PLC
+# side via the driver CLI. Confirms the write path: OPC UA client → server →
+# driver → PLC. This is the direction Test-ServerBridge does NOT cover — a
+# clean Test-ServerBridge only proves reads flow server-ward.
+function Test-OpcUaWriteBridge {
+    param(
+        [Parameter(Mandatory)] $OpcUaCli,
+        [Parameter(Mandatory)] [string]$OpcUaUrl,
+        [Parameter(Mandatory)] [string]$OpcUaNodeId,
+        [Parameter(Mandatory)] $DriverCli,
+        [Parameter(Mandatory)] [string[]]$DriverReadArgs,
+        [Parameter(Mandatory)] [string]$ExpectedValue,
+        [int]$DriverPollDelaySec = 3
+    )
+    Write-Header "OPC UA write bridge"
+
+    $w = Invoke-Cli -Cli $OpcUaCli -Args @(
+        "write", "-u", $OpcUaUrl, "-n", $OpcUaNodeId, "-v", $ExpectedValue)
+    if ($w.ExitCode -ne 0 -or $w.Output -notmatch "Write successful") {
+        Write-Fail "OPC UA client write failed (exit=$($w.ExitCode))"
+        Write-Host $w.Output
+        return @{ Passed = $false; Reason = "opc-ua write failed" }
+    }
+    Write-Info "opc-ua write ok, waiting ${DriverPollDelaySec}s for driver-side apply"
+    Start-Sleep -Seconds $DriverPollDelaySec
+
+    $r = Invoke-Cli -Cli $DriverCli -Args $DriverReadArgs
+    if ($r.ExitCode -ne 0) {
+        Write-Fail "driver-side read failed (exit=$($r.ExitCode))"
+        Write-Host $r.Output
+        return @{ Passed = $false; Reason = "driver read failed" }
+    }
+
+    if ($r.Output -match "Value:\s+$([Regex]::Escape($ExpectedValue))\b") {
+        Write-Pass "PLC-side value equals $ExpectedValue"
+        return @{ Passed = $true }
+    }
+    Write-Fail "PLC-side value mismatch — expected $ExpectedValue"
+    Write-Host $r.Output
+    return @{ Passed = $false; Reason = "reverse-bridge value mismatch" }
+}
+
+# Test 5 — subscribe-sees-change. Start `otopcua-cli subscribe --duration N`
+# in the background, give it ~2s to attach, then write a known value via the
+# driver CLI. After the subscription window closes, assert its captured
+# output mentions the new value. Confirms the OPC UA server is actually
+# pushing data-change notifications for driver-originated changes — not just
+# that a fresh read returns the new value.
+function Test-SubscribeSeesChange {
+    param(
+        [Parameter(Mandatory)] $OpcUaCli,
+        [Parameter(Mandatory)] [string]$OpcUaUrl,
+        [Parameter(Mandatory)] [string]$OpcUaNodeId,
+        [Parameter(Mandatory)] $DriverCli,
+        [Parameter(Mandatory)] [string[]]$DriverWriteArgs,
+        [Parameter(Mandatory)] [string]$ExpectedValue,
+        [int]$DurationSec = 8,
+        [int]$SettleSec = 2
+    )
+    Write-Header "Subscribe sees change"
+
+    # `Start-Job` would spin up a fresh PowerShell runtime and cost 2s+. Use
+    # Start-Process + a temp file instead — it's the same shape Invoke-Cli
+    # uses but non-blocking.
+    $stdout = New-TemporaryFile
+    $stderr = New-TemporaryFile
+    $allArgs = @($OpcUaCli.PrefixArgs) + @(
+        "subscribe", "-u", $OpcUaUrl, "-n", $OpcUaNodeId,
+        "-i", "200", "--duration", "$DurationSec")
+    $proc = Start-Process -FilePath $OpcUaCli.File `
+        -ArgumentList $allArgs `
+        -NoNewWindow -PassThru `
+        -RedirectStandardOutput $stdout.FullName `
+        -RedirectStandardError  $stderr.FullName
+    Write-Info "subscription started (pid $($proc.Id)), waiting ${SettleSec}s to settle"
+    Start-Sleep -Seconds $SettleSec
+
+    $w = Invoke-Cli -Cli $DriverCli -Args $DriverWriteArgs
+    if ($w.ExitCode -ne 0) {
+        Stop-Process -Id $proc.Id -Force -ErrorAction SilentlyContinue
+        Remove-Item $stdout.FullName, $stderr.FullName -ErrorAction SilentlyContinue
+        Write-Fail "driver write during subscribe failed (exit=$($w.ExitCode))"
+        Write-Host $w.Output
+        return @{ Passed = $false; Reason = "driver write failed" }
+    }
+    Write-Info "driver write ok, waiting for subscription window to close"
+
+    # Wait for the subscribe process to exit its --duration timer. Grace
+    # margin on top of the duration in case the first data-change races the
+    # final flush.
+    $proc.WaitForExit(($DurationSec + 5) * 1000) | Out-Null
+    if (-not $proc.HasExited) { Stop-Process -Id $proc.Id -Force }
+
+    $out = (Get-Content $stdout.FullName -Raw) + (Get-Content $stderr.FullName -Raw)
+    Remove-Item $stdout.FullName, $stderr.FullName -ErrorAction SilentlyContinue
+
+    # The subscribe command prints `[timestamp] displayName = value (status)`
+    # per data-change event. We only care that one of those events carried
+    # the new value.
+    if ($out -match "=\s*$([Regex]::Escape($ExpectedValue))\b") {
+        Write-Pass "subscribe saw $ExpectedValue"
+        return @{ Passed = $true }
+    }
+    Write-Fail "subscribe did not observe $ExpectedValue in ${DurationSec}s"
+    Write-Host $out
+    return @{ Passed = $false; Reason = "change not observed on subscription" }
+}
+
 # ---------------------------------------------------------------------------
 # Summary helper — caller passes an array of test results.
 # ---------------------------------------------------------------------------