fix(perf): keep CPU turbo ENABLED in perf mode (was capping at base clock)

Both --extreme and --optimize disabled intel_pstate turbo on the theory
that frequency changes cause jitter. On HWP-capable CPUs (i7-10875H) the
performance governor already holds clocks steady at the top; disabling
turbo only caps all cores at the 2.3GHz base (~64% of clock headroom
thrown away), starving SuperDirt's DSP and causing the very xruns/crackle
it was meant to prevent. Now pins min/max_perf_pct=100 with turbo on.

perf.sh

@@ -187,11 +187,12 @@ optimize_standard() {
   echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor > /dev/null
   echo "✓ CPU governor set to performance"
   
-  # Disable CPU Turbo - can cause audio stutters when frequency changes
+  # Keep CPU Turbo ENABLED — see note in optimize_extreme(). Disabling turbo on
+  # an HWP CPU caps the clock at base and starves SuperDirt's DSP (causes xruns).
   if [ -f "/sys/devices/system/cpu/intel_pstate/no_turbo" ]; then
-    echo "Disabling CPU Turbo (to avoid frequency changes)..."
-    echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
-    echo "✓ CPU Turbo disabled"
+    echo "Ensuring CPU Turbo is enabled..."
+    echo 0 > /sys/devices/system/cpu/intel_pstate/no_turbo
+    echo "✓ CPU Turbo enabled"
   fi
   
   # Apply IRQ thread priorities
@@ -256,16 +257,22 @@ optimize_extreme() {
   echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor > /dev/null
   echo "✓ CPU governor set to performance"
   
-  # Disable CPU Turbo and set min performance
-  echo "Disabling CPU Turbo and frequency scaling..."
+  # Pin CPU to MAXIMUM sustained clock — turbo ENABLED.
+  # NOTE: on HWP-capable CPUs (this i7-10875H has hwp), the performance governor
+  # already holds clocks steady at the top and P-state transitions are near-instant.
+  # Disabling turbo here used to "avoid frequency-change jitter" but it actually
+  # CAPS the CPU at base clock (2.3GHz vs ~4.3GHz turbo) — starving SuperDirt's DSP
+  # and causing the very crackling/xruns it was meant to prevent. Keep turbo ON.
+  echo "Pinning CPU to max clock (turbo enabled)..."
   if [ -f "/sys/devices/system/cpu/intel_pstate/no_turbo" ]; then
-    echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
-    echo "✓ CPU Turbo disabled"
+    echo 0 > /sys/devices/system/cpu/intel_pstate/no_turbo
+    echo "✓ CPU Turbo enabled (full clock headroom available)"
   fi
   
   if [ -d "/sys/devices/system/cpu/intel_pstate" ]; then
+    echo 100 > /sys/devices/system/cpu/intel_pstate/max_perf_pct 2>/dev/null
     echo 100 > /sys/devices/system/cpu/intel_pstate/min_perf_pct 2>/dev/null
-    echo "✓ Intel CPU min performance set to 100%"
+    echo "✓ Intel CPU min/max performance pinned to 100% (no downclocking, turbo reachable)"
   fi
   
   # Disable NMI watchdog (can cause stutters)