Add main entry point + systemd services + integration tests

New files:
  main.py                   - PedalApp: boots all subsystems in order,
                              wires MIDI/footswitch callbacks, graceful
                              teardown reverses boot order
  src/system/config.py      - YAML config loader with deep-merge
                              (separated to avoid hardware deps)
  src/system/services.py    - systemd unit generator for pedal.service
                              + multi-fx-pedal.target
  scripts/install_service.sh - copies project, creates venv, installs
                              + enables service units
  tests/test_integration.py - 41 tests: boot, routing, display sync,
                              teardown, systemd content, CLI, edge cases

Modified:
  tests/conftest.py         - add project root to sys.path

scripts/setup_audio.sh

@@ -25,6 +25,8 @@ ok()    { printf "\e[32m[ OK ] %s\e[0m\n" "$*"; }
 warn()  { printf "\e[33m[WARN] %s\e[0m\n" "$*"; }
 err()   { printf "\e[31m[FAIL] %s\e[0m\n" "$*"; }
 
+NEED_REBOOT=false
+
 # ── Root check ─────────────────────────────────────────────────────
 if [[ $EUID -ne 0 ]]; then
     err "This script must be run as root (sudo ./setup_audio.sh)"