fix: Bug fixes and improvements

- fix(demo-lfo-effects): Fix math.sin() usage (was angle.__sin__())
- feat(pipeline): Add set_effect_intensity() method for runtime effect control
  - Allows changing effect intensity during pipeline execution
  - Returns False if effect not found or intensity out of range
  - Used by LFO modulation demo

The demo-lfo-effects.py script now works correctly with proper
math.sin() usage and the new set_effect_intensity() method provides
a clean API for runtime effect intensity control.

scripts/demo-lfo-effects.py

@@ -14,6 +14,7 @@ Effects modulated:
 The LFO uses a sine wave to oscillate intensity between 0.0 and 1.0.
 """
 
+import math
 import sys
 import time
 from dataclasses import dataclass
@@ -64,7 +65,7 @@ class LFOEffectDemo:
             angle = (
                 (elapsed * effect_cfg.frequency + effect_cfg.phase_offset) * 2 * 3.14159
             )
-            lfo_value = 0.5 + 0.5 * (angle.__sin__())
+            lfo_value = 0.5 + 0.5 * math.sin(angle)
 
             # Scale to intensity range
             intensity = effect_cfg.min_intensity + lfo_value * (