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/plush2/pl_spline.cpp

@@ -0,0 +1,50 @@
+/******************************************************************************
+Plush Version 1.2
+spline.c
+n-th Dimensional Spline Interpolator
+Copyright (c) 1996-2000, Justin Frankel
+******************************************************************************/
+
+#include "plush.h"
+
+void pl_Spline::GetPoint(pl_Float frame, pl_Float *out) {
+  pl_sInt32 i, i_1, i0, i1, i2;
+  pl_Float time1,time2,time3;
+  pl_Float t1,t2,t3,t4,u1,u2,u3,u4,v1,v2,v3;
+  pl_Float a,b,c,d;
+
+  int numKeys=keys.GetSize();
+  pl_Float *keyptrs = keys.Get();
+
+  a = (1-tens)*(1+cont)*(1+bias);
+  b = (1-tens)*(1-cont)*(1-bias);
+  c = (1-tens)*(1-cont)*(1+bias);
+  d = (1-tens)*(1+cont)*(1-bias);
+  v1 = t1 = -a / 2.0; u1 = a; 
+  u2 = (-6-2*a+2*b+c)/2.0; v2 = (a-b)/2.0; t2 = (4+a-b-c) / 2.0; 
+  t3 = (-4+b+c-d) / 2.0; 
+  u3 = (6-2*b-c+d)/2.0; 
+  v3 = b/2.0; 
+  t4 = d/2.0; u4 = -t4;
+
+  i0 = (pl_uInt) frame;
+  i_1 = i0 - 1;
+  while (i_1 < 0) i_1 += numKeys;
+  i1 = i0 + 1;
+  while (i1 >= numKeys) i1 -= numKeys;
+  i2 = i0 + 2;
+  while (i2 >= numKeys) i2 -= numKeys;
+  time1 = frame - (pl_Float) ((pl_uInt) frame);
+  time2 = time1*time1;
+  time3 = time2*time1;
+  i0 *= keyWidth;
+  i1 *= keyWidth;
+  i2 *= keyWidth;
+  i_1 *= keyWidth;
+  for (i = 0; i < keyWidth; i ++) {
+    a = t1*keyptrs[i+i_1]+t2*keyptrs[i+i0]+t3*keyptrs[i+i1]+t4*keyptrs[i+i2];
+    b = u1*keyptrs[i+i_1]+u2*keyptrs[i+i0]+u3*keyptrs[i+i1]+u4*keyptrs[i+i2];
+    c = v1*keyptrs[i+i_1]+v2*keyptrs[i+i0]+v3*keyptrs[i+i1];
+    *out++ = a*time3 + b*time2 + c*time1 + keyptrs[i+i0];
+  }
+}