Implement sfx_play() sound effect subroutine

[jonathanpeppers]

Problem

sfx_play(byte sound, byte channel) is declared in NESLib but has no 6502 implementation in BuiltInSubroutines.cs. The crypto sample has 6 sfx_play calls that are commented out as a workaround.

// These are all commented out because sfx_play has no implementation:
// sfx_play(3, 2);  // pickup sound
// sfx_play(4, 2);  // level complete
// sfx_play(5, 3);  // attack sound
// sfx_play(1, 0);  // damage taken
// sfx_play(2, 3);  // enemy killed
// sfx_play(0, 0);  // item placed

What's Needed

A 6502 implementation of sfx_play that plays sound effects through the NES APU. This is typically provided by a sound engine like FamiTone2 or Pently.

cc65/neslib implementation

In the original cc65 neslib, sfx_play is part of the FamiTone2 sound engine. It:

1. Takes a sound effect index in A and channel (0-3) in X
2. Loads the sound effect data pointer from a table
3. Starts playback on the specified APU channel
4. The NMI handler calls sfx_update each frame to advance the sound

References

• FamiTone2 source: https://github.com/Shiru/FamiTone2
• NESdev Wiki APU: https://www.nesdev.org/wiki/APU
• neslib sfx_play usage: https://github.com/clbr/neslib

Implementation Options

1. Port FamiTone2's sfx routines to BuiltInSubroutines.cs -- most compatible with existing neslib samples
2. Stub implementation -- play a simple beep on the specified channel (minimal but functional)
3. No-op -- emit an RTS stub so the transpiler doesn't crash but no sound plays

Context

The crypto sample (PR #211) has 6 sound effects for gameplay feedback. Without sfx_play, the game is silent.

[jonathanpeppers]

Better approach: use an extern .s file with FamiTone2's sfx routines, same pattern as the music and irq samples.

// In Program.cs -- declare as extern
static extern void sfx_play(byte sound, byte channel);

// Usage (unchanged):
sfx_play(3, 2);

; In sfx_engine.s -- FamiTone2 sfx routines
.segment "CODE"
_sfx_play:
    ; A = sound index, X = channel
    ; ... FamiTone2 sfx_play implementation ...
    rts

The transpiler already supports this pattern via ExternMethods + Ca65Assembler. The music sample does exactly this with _famitone_init, _famitone_update, etc.

FamiTone2 sfx source (MIT licensed): https://github.com/Shiru/FamiTone2/blob/master/famitone2.s -- the FamiToneSfxPlay routine and FamiToneSfxUpdate (called from NMI) are the relevant parts.