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/giflib/AUTHORS

@@ -0,0 +1,36 @@
+Lennie Araki
+	Windows code providing a nicer interface and example program
+
+Michael Brown <michael_brown_uk[AT]hotmail.com>
+        callbacks to write data via user defined function
+
+Daniel Eisenbud <eisenbud[AT]google.com>
+	Fixes for crashes with invalid gif files and double freeing of
+	colormaps
+
+Gershon Elber
+	original giflib code
+
+Marc Ewing <marc[AT]redhat.com>
+        spec file (for rpms) updates
+
+Toshio Kuratomi <toshio[AT]tiki-lounge.com>
+        uncompressed gif writing code
+        autoconf/automake process
+        current maintainer
+
+marek <marwaw[AT]users.sourceforge.net>
+	Gif initialization fix
+	windows build code
+
+Peter Mehlitz <peter[AT]transvirtual.com>
+        callbacks to read data from arbitrary sources (like libjpeg/libpng)
+
+Dick Porter <dick[AT]cymru.net>
+        int/pointer fixes for Alpha
+
+Eric Raymond <esr[AT]snark.thyrsus.com>
+        long time maintainer of giflib code
+
+Georg Schwarz <geos[AT]epost.de>
+	IRIX fixes