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/build.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# Builds extension-reaper-macos as a REAPER extension .dylib.
+set -e
+cd "$(dirname "$0")"
+
+OUT="reaper_extension-reaper-macos.dylib"
+
+clang++ \
+  -std=c++17 \
+  -fvisibility=hidden \
+  -dynamiclib \
+  -arch arm64 \
+  -Ivendor/reaper-sdk/sdk \
+  -Ivendor/reaper-sdk/WDL \
+  -o "$OUT" \
+  src/main.cpp
+
+echo "Built $OUT"