Local NI-VISA backend: SRQ + AssertTrigger parity

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Context

ADR 0041
implemented Service Request (SRQ) and AssertTrigger on the
HiSlip and VXI-11 backends in v0.1.0, but the Local NI-VISA
backend lacks them.

CHANGELOG.md → Known limitations:

Local backend SRQ / AssertTrigger deferred (ADR 0041) — HiSlip /
VXI-11 backends are at parity; Local needs IVI.NET reflection
follow-up.

Why deferred

The Local backend talks to NI-VISA via IVI.NET (Ivi.Visa.dll),
which exposes SRQ events through IMessageSession.ServiceRequest +=.
Wiring this up requires:

• Resolving the right interface (depends on the IVI.NET version
  the user has installed; Ivi.Visa.IMessageSession may have
  diverged across versions)
• Mapping the SRQ event into the same IIviBackend callback
  shape the HiSlip / VXI-11 backends already use
• Handling the async / synchronization-context lifetime
  correctly (SRQ event fires on a callback thread, not the
  user's thread)

Acceptance

• IIviBackend.WaitForSrqAsync works on Local NI-VISA backend
  identically to HiSlip / VXI-11.
• IIviBackend.AssertTriggerAsync triggers *TRG on Local.
• Architecture test confirms all backends implement the SRQ /
  trigger members.
• ADR 0041 updated in place: remove "Local deferred" caveat.