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-all.ps1

@@ -1,4 +1,4 @@
-#Requires -Version 7.0
+#Requires -Version 7.0
 <#
 .SYNOPSIS
   Runs every scripts/e2e/test-*.ps1 and tallies PASS / FAIL / SKIP.