Add non-blocking TryBeginInvoke and clarify the realtime threading model

[matt-edmondson]

Summary

Implements upstream roadmap item #4 from the .NET VST3 effects host plan: clarify Invoker's realtime story and add a non-blocking, allocation-free marshalling path for non-realtime producers.

TryBeginInvoke(Action)

A fire-and-forget counterpart to Invoke/InvokeAsync:

• Non-blocking and allocation-free — no Task is created and the caller never blocks waiting for the owner thread to pump.
• Returns false when the bounded queue is full (the action is dropped) rather than growing unboundedly — predictable memory behaviour.
• Runs immediately when called from the owner thread; otherwise queued and drained in FIFO order by DoInvokes, alongside the existing Task queue.
• New Invoker(int beginInvokeCapacity) constructor sizes the queue (default 1024).

BoundedMpscQueue<T> (internal)

A bounded, lock-free queue implementing Dmitry Vyukov's bounded MPMC algorithm (per-cell sequence numbers, single CAS per op). Safe for any number of concurrent producers; pre-allocates its cells and allocates nothing per enqueue/dequeue.

Documentation

Adds an XML-doc threading section making explicit that:

• Invoker marshals work onto the owner thread; it doesn't make arbitrary code thread-safe.
• The blocking/async paths allocate a Task and may block, so they must never be called from a hard real-time audio thread. Audio→UI telemetry should flow through a dedicated SPSC ring buffer (see the companion Containers PR) rather than the invoker.

Testing

Added TryBeginInvokeTests: null-guard, immediate owner-thread execution, cross-thread queue-until-pump, FIFO ordering, full-queue rejection, and a 4-producer × 10k concurrent delivery test. Full suite: 16/16 passing locally (net10.0).

Note: the local sandbox's .NET SDK (10.0.108) predates the Roslyn the pinned ktsu.Sdk analyzers (2.8.0) require, so tests were run via the standard MSTest runner against net10.0. CI runs the full multi-target build with analyzers and style enforcement.

🤖 Part of the ktsu VST plugin upstream-enablement work.

https://claude.ai/code/session_01JQ3bQMuSYuumAnQZaYUKP7

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