RT performance tuning: IRQ affinity, chrt, xrun tracking, reference doc

- Add USB audio IRQ affinity pinning to core 3 in main.py
- Add enable_xrun_tracking() to AudioSystem for kernel-level diagnostics
- Wrap Python process with chrt -f 80 in systemd service template
- Add LimitSIGPENDING=128 for signal queue depth
- Create scripts/rt-tune.sh — comprehensive RT tuning startup script
  (IRQ affinity, CPU governor, C-states, ALSA limits, xrun_debug)
- Create docs/rt-performance-tuning.md — reference doc with all
  tuning knobs, measurement tools, and systematic procedure

Targets: <12ms RT latency (8ms ideal), zero xruns, CPU <40% at 512/48k

main.py

@@ -194,6 +194,12 @@ class PedalApp:
             except Exception:
                 pass
 
+            # Enable kernel-level xrun tracking for diagnostics
+            try:
+                self.audio_system.enable_xrun_tracking()
+            except Exception:
+                pass
+
             if self.audio_config.jack_enabled:
                 self.audio_system.start_jack(timeout=10)
             else:
@@ -839,6 +845,48 @@ def main() -> int:
     except Exception as exc:
         logger.warning("Could not set CPU governor (non-root?): %s", exc)
 
+    # Pin USB audio IRQ to a dedicated core for stable RT audio
+    # On RPi 4B, cores 0-2 handle kernel/general interrupts;
+    # pinning USB audio to core 3 isolates it from that noise.
+    try:
+        import glob as _glob
+        # Find the USB audio interface IRQ
+        _usb_irq: str | None = None
+        # Strategy 1: xhci-hcd (USB 3.0 controller on Pi 4B)
+        for _proc_dir in _glob.glob("/proc/irq/[0-9]*"):
+            try:
+                _name = (_glob.glob(f"{_proc_dir}/name")[0] if _glob.glob(f"{_proc_dir}/name") else None)
+                if _name:
+                    _irq_name = open(_name).read().strip()
+                    if "xhci" in _irq_name or "dwc" in _irq_name:
+                        _usb_irq = os.path.basename(_proc_dir)
+                        break
+            except (OSError, PermissionError, IndexError):
+                continue
+        if _usb_irq:
+            # Pin to core 3 (smp_affinity mask = 0x8)
+            _aff_path = f"/proc/irq/{_usb_irq}/smp_affinity"
+            _aff_list_path = f"/proc/irq/{_usb_irq}/smp_affinity_list"
+            with open(_aff_path, "w") as f:
+                f.write("8")
+            with open(_aff_list_path, "w") as f:
+                f.write("3")
+            # Move all other IRQs away from core 3
+            for _proc_dir in _glob.glob("/proc/irq/[0-9]*"):
+                _irq_num = os.path.basename(_proc_dir)
+                if _irq_num == _usb_irq:
+                    continue  # Skip our USB audio IRQ
+                try:
+                    with open(f"{_proc_dir}/smp_affinity", "w") as f:
+                        f.write("7")  # cores 0,1,2
+                except (OSError, PermissionError):
+                    continue
+            logger.info("USB audio IRQ %s pinned to core 3 for RT stability", _usb_irq)
+        else:
+            logger.info("No USB audio IRQ found — skipping IRQ affinity (non-critical)")
+    except Exception as exc:
+        logger.warning("Could not set IRQ affinity (non-root?): %s", exc)
+
     # Disable Python garbage collector to prevent 10-50ms GC pauses in
     # the real-time audio callback. At ~500 numpy allocs/sec in the
     # pipeline, default GC (threshold=700) triggers every ~1.4s,