Add bare-minimum extension-reaper-macos REAPER extension skeleton

C/C++ extension built directly against the vendored Cockos reaper-sdk
headers (no Rust/reaper-rs wrapper layer). Loads, registers a test
action, and confirms via REAPER's console — confirmed working in
REAPER 2026-07-15.

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

extension-reaper-macos/src/main.cpp

@@ -0,0 +1,50 @@
+// extension-reaper-macos — bare-minimum REAPER extension.
+//
+// Verifies the extension loads and is recognized by REAPER: prints a
+// confirmation line to the REAPER console on load, and registers one test
+// action ("extension-reaper-macos: Hello") in the Actions list.
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_ShowConsoleMsg
+#define REAPERAPI_IMPLEMENT
+
+#include "reaper_plugin.h"
+#include "reaper_plugin_functions.h"
+
+static int g_hello_cmd_id = 0;
+
+static bool HookCommand2(KbdSectionInfo *sec, int command, int val, int val2, int relmode, HWND hwnd)
+{
+  if (command == g_hello_cmd_id)
+  {
+    ShowConsoleMsg("[extension-reaper-macos] Hello action triggered\n");
+    return true;
+  }
+  return false;
+}
+
+extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(REAPER_PLUGIN_HINSTANCE hInstance, reaper_plugin_info_t *rec)
+{
+  if (!rec)
+    return 0; // REAPER is unloading us
+
+  if (rec->caller_version != REAPER_PLUGIN_VERSION)
+    return 0;
+
+  if (REAPERAPI_LoadAPI(rec->GetFunc) != 0)
+    return 0; // a required API function was missing
+
+  custom_action_register_t action = {
+    0, // main section
+    "EXTENSION_REAPER_MACOS_HELLO",
+    "extension-reaper-macos: Hello",
+    NULL,
+  };
+  g_hello_cmd_id = rec->Register("custom_action", &action);
+
+  rec->Register("hookcommand2", (void *)HookCommand2);
+
+  ShowConsoleMsg("[extension-reaper-macos] loaded successfully\n");
+
+  return 1;
+}