Add SFX sound synthesis improvement guide

Document 12 easy improvements to the sound pipeline covering
waveform bug fixes, exponential ADSR envelopes, click prevention,
soft saturation, tremolo, configurable pulse duty cycle, DC offset
removal, and harmonics normalization. Each rated by impact/effort
with code-level guidance.

https://claude.ai/code/session_01XDAnX2yoseGsbSS2YGCirn

Author: claude