Galaxy IPC unblock — live dev-box E2E path

Three root-cause fixes to get an elevated dev-box shell past session open
through to real MXAccess reads:

1. PipeAcl — drop BUILTIN\Administrators deny ACE. UAC's filtered token
   carries the Admins SID as deny-only, so the deny fired even from
   non-elevated admin-account shells. The per-connection SID check in
   PipeServer.VerifyCaller remains the real authorization boundary.

2. PipeServer — swap the Hello-read / VerifyCaller order. ImpersonateNamedPipeClient
   returns ERROR_CANNOT_IMPERSONATE until at least one frame has been read
   from the pipe; reading Hello first satisfies that rule. Previously the
   ACL deny-first path masked this race — removing the deny ACE exposed it.

3. GalaxyIpcClient — add a background reader + single pending-response
   slot. A RuntimeStatusChange event between OpenSessionRequest and
   OpenSessionResponse used to satisfy the caller's single ReadFrameAsync
   and fail CallAsync with "Expected OpenSessionResponse, got
   RuntimeStatusChange". The reader now routes response kinds (and
   ErrorResponse) to the pending TCS and everything else to a handler the
   driver registers in InitializeAsync. The Proxy was already set up to
   raise managed events from RaiseDataChange / RaiseAlarmEvent /
   OnHostConnectivityUpdate — those helpers had no caller until now.

4. RedundancyPublisherHostedService — swallow BadServerHalted while
   polling host.Server.CurrentInstance. StandardServer throws that code
   during startup rather than returning null, so the first poll attempt
   crashed the BackgroundService (and the host) before OnServerStarted
   ran. This race was latent behind the Galaxy init failure above.

Updates docs that described the Admins deny ACE + mandatory non-elevated
shells, and drops the admin-skip guards from every Galaxy integration +
E2E fixture that had them (IpcHandshakeIntegrationTests, EndToEndIpcTests,
ParityFixture, LiveStackFixture, HostSubprocessParityTests).

Adds GalaxyIpcClientRoutingTests covering the router's
request/response match, ErrorResponse, event-between-call, idle event,
and peer-close paths.

Verified live on the dev box against the p7-smoke cluster (gen 6):
driver registered=1 failedInit=0, Phase 7 bridge subscribed, OPC UA
server up on 4840, MXAccess read round-trip returns real data with
Status=0x00000000.

Task #112 — partial: Galaxy live stack is functional end-to-end. The
supplied test-galaxy.ps1 script still fails because the UNS walker
encodes TagConfig JSON as the tag's NodeId instead of the seeded TagId
(pre-existing; separate issue from this commit).

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

docs/ServiceHosting.md

@@ -83,7 +83,7 @@ The host spins up `StaPump` (the STA thread with message pump), creates the MXAc
 
 ### Pipe security
 
-`PipeServer` builds a `PipeAcl` from the provided `SecurityIdentifier` + uses `NamedPipeServerStream` with `maxNumberOfServerInstances: 1`. The handshake requires a matching shared secret in the first Hello frame; callers whose SID doesn't match `OTOPCUA_ALLOWED_SID` are rejected before any frame is processed. **By design the pipe ACL denies BUILTIN\Administrators** — live smoke tests must therefore run from a non-elevated shell that matches the allowed principal. The installed dev host (`OtOpcUaGalaxyHost`) runs as `dohertj2` with the secret at `.local/galaxy-host-secret.txt`.
+`PipeServer` builds a `PipeAcl` from the provided `SecurityIdentifier` + uses `NamedPipeServerStream` with `maxNumberOfServerInstances: 1`. The handshake requires a matching shared secret in the first Hello frame; callers whose SID doesn't match `OTOPCUA_ALLOWED_SID` are rejected before any frame is processed via `NamedPipeServerStream.RunAsClient` + a SID comparison against the configured allow list. The DACL grants `ReadWrite | Synchronize` only to the allowed SID and denies `LocalSystem`. The installed dev host (`OtOpcUaGalaxyHost`) runs as `dohertj2` with the secret at `.local/galaxy-host-secret.txt`.
 
 ### Installation