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

src/system/audio.py

@@ -17,6 +17,7 @@ import subprocess
 import time
 from dataclasses import dataclass, field
 from pathlib import Path
+from collections.abc import Callable
 from typing import Optional
 
 logger = logging.getLogger(__name__)
@@ -143,6 +144,10 @@ class AudioSystem:
 
     def __init__(self, config: Optional[AudioConfig] = None) -> None:
         self.config = config or AudioConfig()
+        self._tempo_bpm: float = 120.0
+        self._tempo_source: str = "default"  # "default", "midi_clock", "manual"
+        self._midi_clock_enabled: bool = False
+        self._tempo_callback: Optional[Callable[[float], None]] = None
 
     # ──────────────────────────────────────────────────────────────
     # I2S overlay management
@@ -580,6 +585,70 @@ WantedBy=multi-user.target
             return False
 
 
+    # ──────────────────────────────────────────────────────────────
+    # MIDI clock sync (tempo integration for time-based FX)
+    # ──────────────────────────────────────────────────────────────
+
+    @property
+    def tempo_bpm(self) -> float:
+        """Current tempo in BPM for time-based effects (delay, reverb)."""
+        return self._tempo_bpm
+
+    @tempo_bpm.setter
+    def tempo_bpm(self, bpm: float) -> None:
+        """Set tempo manually (overrides MIDI clock)."""
+        self._tempo_source = "manual"
+        self._tempo_bpm = max(20.0, min(300.0, bpm))
+        if self._tempo_callback:
+            self._tempo_callback(self._tempo_bpm)
+        logger.info("Tempo set manually: %.1f BPM", self._tempo_bpm)
+
+    @property
+    def tempo_source(self) -> str:
+        return self._tempo_source
+
+    def set_tempo_callback(self, callback: Callable[[float], None]) -> None:
+        """Register a callback for tempo changes.
+
+        Args:
+            callback: Called with new BPM value when tempo changes.
+        """
+        self._tempo_callback = callback
+
+    def set_tempo_from_midi_clock(self, bpm: float) -> None:
+        """Update tempo from MIDI clock.
+
+        Called by MIDIHandler's clock callback. Only updates if MIDI
+        clock sync is enabled.
+
+        Args:
+            bpm: Detected BPM from MIDI clock (20-300).
+        """
+        if not self._midi_clock_enabled:
+            return
+        bpm = max(20.0, min(300.0, bpm))
+        if abs(self._tempo_bpm - bpm) > 0.5 or self._tempo_source != "midi_clock":
+            self._tempo_bpm = bpm
+            self._tempo_source = "midi_clock"
+            if self._tempo_callback:
+                self._tempo_callback(bpm)
+            logger.debug("Tempo synced from MIDI clock: %.1f BPM", bpm)
+
+    def enable_midi_clock_sync(self, enabled: bool = True) -> None:
+        """Enable or disable MIDI clock sync.
+
+        When enabled, tempo follows MIDI clock. When disabled, tempo
+        stays at the last value but source reverts to manual.
+
+        Args:
+            enabled: Whether to follow MIDI clock.
+        """
+        self._midi_clock_enabled = enabled
+        if not enabled and self._tempo_source == "midi_clock":
+            self._tempo_source = "manual"
+        logger.info("MIDI clock sync %s", "enabled" if enabled else "disabled")
+
+
 # ═══════════════════════════════════════════════════════════════════
 # Internal helpers
 # ═══════════════════════════════════════════════════════════════════