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/zlib/gzclose.c

@@ -0,0 +1,23 @@
+/* gzclose.c -- zlib gzclose() function
+ * Copyright (C) 2004, 2010 Mark Adler
+ * For conditions of distribution and use, see copyright notice in zlib.h
+ */
+
+#include "gzguts.h"
+
+/* gzclose() is in a separate file so that it is linked in only if it is used.
+   That way the other gzclose functions can be used instead to avoid linking in
+   unneeded compression or decompression routines. */
+int ZEXPORT gzclose(gzFile file) {
+#ifndef NO_GZCOMPRESS
+    gz_statep state;
+
+    if (file == NULL)
+        return Z_STREAM_ERROR;
+    state = (gz_statep)file;
+
+    return state->mode == GZ_READ ? gzclose_r(file) : gzclose_w(file);
+#else
+    return gzclose_r(file);
+#endif
+}