feature: add apple silicon native macOS support (#81)

* feature: add apple silicon native macOS support - #81

* (macos): Fix crash

This fixes a crash when viewing the rear camera

* fix(macos): keep interpolated presentation on main thread

* fix(macos): supply Retro-WFC payload during setup

* perf(windows): compile out flat-memory fallback check

* remove duplicate smoke test

* test(macos): name and focus host platform tests

* fix(macos): validate Retro-WFC payload cache

* fix(payload): preserve staged file access failures

* Limit flat-page checks to variable-page hosts

---------

Co-authored-by: patchzyy <64382339+patchzyy@users.noreply.github.com>

runtime/include/host_context.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <cstddef>
+
+// HostContext is the deliberately small boundary between the guest scheduler
+// and the host's cooperative-context facility. Windows uses native Fibers and
+// Linux uses libco; macOS AArch64 uses the local assembly backend because it
+// must preserve Darwin's platform-reserved x18 register, which libco's AArch64
+// backend does not save. Its handles are only valid on the thread that
+// initialized the scheduler.
+namespace HostContext {
+
+using Handle = void*;
+using Entry = void (*)(void*);
+
+bool InitializeScheduler(Handle* scheduler);
+void ShutdownScheduler(Handle scheduler);
+
+Handle Create(std::size_t stackSize, Entry entry, void* argument);
+void Destroy(Handle context);
+bool IsCurrent(Handle context);
+void Switch(Handle target);
+
+} // namespace HostContext