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/test_a.eel

@@ -0,0 +1,22 @@
+sum=0;
+a= 1;
+b = .1;
+c= .2;
+d = .5;
+e = 4;
+f = -0.2;
+g = 0.3;
+h = .37;
+
+loop(62000,
+  i = 0;
+  sum=0.0;
+  loop(8192,
+    sum += i*(3+ a * b + c*d + e * (f - b*(-c)  + g*(e-g*(-g)*g)) );
+    sum2 += (a*b)+(a*b)+((a*b)*((b*c)*((h*i)*(f*g))));
+    i += 0.001;
+    i2+=1;
+  );
+);
+printf("sum error %.18f %.18f\n", sum - 245333.476966294896556064, sum2 - 101488440.519564077258110046);
+printf("sum error per inst %.18f %.18f\n", (sum - 245333.476966294896556064) / i, (sum2 - 101488440.519564077258110046)/i2);