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/sinewave_text.eel

@@ -0,0 +1,17 @@
+width = 80;
+lx=-1;
+loop(1000000,
+  x = ((1+sin(pos))*0.5 * width)|0;
+  lx >= -1 && lx!=x ? (
+    loop(min(x,lx), printf(" "));
+    loop(abs(x-lx), printf("*"));
+    loop(width-max(x,lx), printf(" "));
+  ) : (
+    loop(x, printf(" "));
+    printf("*");
+    loop(width-1-x, printf(" "));
+  );
+  lx=x;
+  printf("\n");
+  pos += 0.25 + sin(pos*0.32)*0.2;
+);