sfx-maker: sfxc - sfxr-style .sfx text presets -> 16-bit WAV synthesis

- waves: square (duty), saw, sine, triangle, pitched noise (seeded, deterministic)
- envelope attack/sustain/decay, freq slide, vibrato, arpeggio jump,
  one-pole low/high-pass filters, clamped output
- presets blip/coin/explosion/hurt/jump/laser/powerup with seeded variants;
  preset writes editable .sfx or renders .wav directly
- go tests: parse/validate, determinism, per-preset RMS, WAV header roundtrip

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmdG9GqfSWCzts7AkDwRDh

sfx-maker/examples/powerup.sfx

@@ -0,0 +1,12 @@
+# powerup preset (seed 0) — edit freely, then: sfxc build examples/powerup.sfx
+sfx: powerup
+wave: square
+volume: 0.7
+attack: 0.01
+sustain: 0.25
+decay: 0.25
+freq: 220
+freq-slide: 700
+duty: 0.4
+vibrato-depth: 25
+vibrato-rate: 9