Replace illwill with custom canvas — zero dependencies

- Custom terminal canvas: one buffer, one write() per frame
- Threaded audio via Nim channels (20fps → 60fps)
- 6 phosphor palettes (green, amber, cyan, blue, white, red)
- All tuning constants at top of osc.nim
- No illwill dependency, binary 352KB → 200KB
- CRT effects use same palette as trace

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

osc.nimble

@@ -10,4 +10,3 @@ bin           = @["osc"]
 # Dependencies
 
 requires "nim >= 2.2.8"
-requires "illwill >= 0.4.1"