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/swell/swell_resgen.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+DIR="$(dirname $0)"
+if [ "v$DIR" = "v" ]; then DIR=. ; fi
+
+"$DIR/swell_resgen.pl" $*