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/eel2/scripts/gfx_test.eel

@@ -0,0 +1,24 @@
+gfx_init("vis",1024,768);
+
+gfx_clear=-1;
+while ((c=gfx_getchar())!=27 && c >= 0)
+(
+  c == ' ' ? (mode += 1) >= 2 ? mode=0;
+  t=time_precise();
+  zsc = 0.01*cos(t*0.73);
+//  gfx_x=gfx_y=0; gfx_blurto(gfx_w,gfx_h);
+  gfx_blit(-1,0,0.3*(sin(t*0.3)^2),gfx_w*zsc,gfx_h*zsc, gfx_w*(1-2*zsc),gfx_h*(1-2*zsc), 0,0,gfx_w,gfx_h);
+  gfx_r=(cos(t)+1.0)*0.5;
+  gfx_g=(cos(t*1.74)+1.0)*0.5;
+  gfx_b=(cos(t*1.2+0.56)+1.0)*0.5;
+  gfx_a=0.15;
+  sz=gfx_w*0.03;
+  loop(20,
+   mode == 1 ?     
+     gfx_circle(rand(gfx_w-sz),rand(gfx_h-sz),sz,1) : 
+     gfx_rect(rand(gfx_w-sz),rand(gfx_h-sz),sz,sz);
+  );
+
+
+  gfx_update();
+);