refactor(audio): extract AddonContext + NapiHelpers, guard raw N-API args (phase 6)

AddonContext (src/audio/addon/): engine/vstHost lifetime + snapshot rule,
the JUCE message thread with the macOS no-pump fork quarantined into ONE
file, the shutdown latch (exposed as isShuttingDown), doShutdown with a UI
teardown hook (NodeAddon points it at the editor-window nuke, #56), and the
pending-async-load registry. NodeAddon keeps using-declarations so the
binding bodies are unchanged. Also fixes SetBackingSpeed's bare `engine`
dereference — the one binding that dodged the file's own snapshot rule.

NapiHelpers (typed extractors argInt/argSlotId/argFiniteFloat/argBool/
argMidiChannel/argMidiByte) + rewrites of the unguarded bindings — the
deep-read §2 fix, done once: SetParameter/SetBypass/RemoveProcessor/
MoveProcessor/SetMultiBypass/SendMidiToSlot/SetGain plus the St-1 routing
quartet (SetPan/SetPostGain/SetBranch/SetBranchSrc). NaN slot ids no longer
coerce to slot 0; MIDI channel/program are range-checked before JUCE.

New gate: tests/napi-arg-fuzz.test.js — table-driven garbage (NaN/Inf/
negative/string/missing/object) against the real addon; chain state must be
byte-identical after the storm and a valid call must still apply.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/audio/addon/AddonContext.h

@@ -0,0 +1,66 @@
+#pragma once
+
+// AddonContext — engine/vstHost lifetime, the JUCE message thread, the
+// shutdown latch, and the pending-async-load registry (TLC plan phase 6 /
+// §3.1). Moved verbatim from NodeAddon.cpp; this quarantines the JUCE_MAC
+// platform fork (no dispatch loop — see startJuceMessageThread) into ONE
+// file instead of a branch inside every load path.
+//
+// engine / vstHost — shared_ptr (not unique_ptr) so worker threads can take
+// a stable snapshot that keeps the object alive for the duration of their
+// work, even if the message thread reassigns the global mid-operation. This
+// matters most for the async VST load: createPluginInstanceAsync's JUCE
+// continuation must not have VSTHost / its formatManager torn out from
+// under it mid-load.
+//
+// Enforced rule: every *dereference* of engine / vstHost goes through a
+// local snapshot (snapshotEngine / snapshotVstHost). The only code touching
+// the bare globals is the snapshot helpers and the mutex-guarded writes in
+// initialize / doShutdown.
+
+#include "../AudioEngine.h"
+#include "../VSTHost.h"
+
+#include <juce_events/juce_events.h>
+
+#include <atomic>
+#include <functional>
+#include <memory>
+#include <mutex>
+
+namespace slopsmith::addon {
+
+std::shared_ptr<AudioEngine> snapshotEngine();
+std::shared_ptr<VSTHost> snapshotVstHost();
+
+// Start the pump + create engine/vstHost on the message thread (inline on
+// macOS). `uiTeardownHook` runs on the message thread at the START of
+// shutdown, BEFORE engine.reset() frees the processors — NodeAddon points it
+// at destroyAllPluginEditorWindowsOnMessageThread (use-after-free; #56).
+void initialize(std::function<void()> uiTeardownHook);
+void doShutdown();
+
+// Dispatch `func` on the JUCE message thread and wait (bounded 15 s).
+// macOS: executes inline on the caller thread — no background pump exists
+// (AppKit owns the real main thread; see the fork note in the .cpp).
+void dispatchOnMessageThreadImpl(std::function<void()> func);
+template <typename Func>
+inline void dispatchOnMessageThread(Func&& func)
+{
+    dispatchOnMessageThreadImpl(std::function<void()>(std::forward<Func>(func)));
+}
+
+// Pending-async-load registry: LoadVSTWorker / LoadPresetWorker block on a
+// WaitableEvent until the message-thread continuation fires; doShutdown
+// signals every registered event so no worker waits forever once the pump
+// is gone.
+// Whether doShutdown has begun (acquire). The load workers gate on this
+// after registering their pending event, catching the register-vs-shutdown
+// race in both directions.
+bool isShuttingDown();
+
+void registerPendingLoad(std::shared_ptr<juce::WaitableEvent> evt);
+void unregisterPendingLoad(const std::shared_ptr<juce::WaitableEvent>& evt);
+void cancelAllPendingLoads();
+
+} // namespace slopsmith::addon