Reorganize extension-reaper-macos into per-module subfolders, add hello module

Splits socket/tracking into their own subdirectories and pulls the hello
action registration out into its own module, matching the one-module-per-
folder pattern; build.sh now outputs into build/macos instead of the repo root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

extension-reaper-macos/build.sh

@@ -3,7 +3,11 @@
 set -e
 cd "$(dirname "$0")"
 
-OUT="reaper_extension-reaper-macos.dylib"
+NAME="reaper_extension-reaper-macos.dylib"
+OUT_DIR="build/macos"
+OUT="$OUT_DIR/$NAME"
+
+mkdir -p "$OUT_DIR"
 
 clang++ \
   -std=c++17 \
@@ -12,13 +16,15 @@ clang++ \
   -arch arm64 \
   -Ivendor/reaper-sdk/sdk \
   -Ivendor/reaper-sdk/WDL \
+  -Isrc \
   -o "$OUT" \
   src/main.cpp \
-  src/tracking.cpp \
-  src/socket.cpp
+  src/hello/hello.cpp \
+  src/tracking/tracking.cpp \
+  src/socket/socket.cpp
 
 echo "Built $OUT"
 
-DEST="$HOME/Library/Application Support/REAPER/UserPlugins/$OUT"
+DEST="$HOME/Library/Application Support/REAPER/UserPlugins/$NAME"
 cp "$OUT" "$DEST"
 echo "Copied to $DEST (restart REAPER to reload)"