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/vendor/reaper-sdk/WDL/lice/test/English.lproj/MainMenu.nib/classes.nib

@@ -0,0 +1,7 @@
+{
+    IBClasses = (
+        {CLASS = Controller; LANGUAGE = ObjC; SUPERCLASS = NSObject; }, 
+        {CLASS = FirstResponder; LANGUAGE = ObjC; SUPERCLASS = NSObject; }
+    ); 
+    IBVersion = 1; 
+}

extension-reaper-macos/vendor/reaper-sdk/WDL/lice/test/English.lproj/MainMenu.nib/info.nib

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IBDocumentLocation</key>
+	<string>110 86 356 240 0 0 1440 878 </string>
+	<key>IBEditorPositions</key>
+	<dict>
+		<key>29</key>
+		<string>109 299 338 44 0 0 1440 878 </string>
+	</dict>
+	<key>IBFramework Version</key>
+	<string>446.1</string>
+	<key>IBOpenObjects</key>
+	<array>
+		<integer>29</integer>
+	</array>
+	<key>IBSystem Version</key>
+	<string>8R2232</string>
+</dict>
+</plist>