fluidsyX: resume macros waiting on keyOff/sampleEnd via FluidSynth voice callbacks

[Copilot]

fluidsyX had no mechanism to detect voice noteoff or sample completion, so CmdWaitTicks with keyOff or sampleEnd flags would block macro execution indefinitely. FluidSynth PR #1778 (≥ 2.5.4) adds fluid_voice_set_callback with FLUID_VOICE_CALLBACK_NOTEOFF / FLUID_VOICE_CALLBACK_FINISHED reasons, enabling these signals.

Additionally, two pre-existing bugs in CmdWaitTicks::DoFluid prevented the wait from ever terminating even when conditions were met.

New: voice callback infrastructure (FLUID_VERSION_AT_LEAST(2, 5, 4))

• VoiceCallbackData — carries app, macroId, voiceId (for self-removal from map on FINISHED), stored in a new std::map<unsigned int, VoiceCallbackData> voiceCallbacks on FluidsyXApp. std::map stable-reference guarantee makes passing &entry to FluidSynth safe across subsequent inserts.
• voiceStateCallback — sets keyoffReceived on NOTEOFF, sampleEndReceived on FINISHED. For indefinite waits only (inIndefiniteWait), calls scheduleImmediateResume to unblock the macro via fluid_sequencer_send_now. Finite timed waits only get the flag set; their existing timers fire and check it. The FINISHED handler also erases the voiceCallbacks entry.
• MacroExecContext::selfId — macroId is now pre-assigned in enqueueSoundMacro before the command loop and stored on the context, making it available inside dummy_preset_noteon (called synchronously from fluid_synth_start) when registering the callback.

Bug fixes in CmdWaitTicks::DoFluid

• Missing ctx.pc++: both pre-checks (keyOff already received, sampleEnd already received) returned 0 without advancing the program counter, causing the outer loop to re-execute the same wait instruction forever.
• if(v) guard on keyoff pre-check: after releaseVoice() sets ctx.voiceId = 0, findVoiceById returns nullptr and the pre-check was silently skipped. Fixed to treat v == nullptr as "not sustained":

// Before
if (v) {
    if (ctx.waitingKeyoff && ctx.keyoffReceived &&
        !fluid_voice_is_sustained(v) && !fluid_voice_is_sostenuto(v)) {
        ctx.waitingKeyoff = false;
        return 0;  // missing ctx.pc++
    }
}

// After
if (ctx.waitingKeyoff && ctx.keyoffReceived &&
    (!v || (!fluid_voice_is_sustained(v) && !fluid_voice_is_sostenuto(v)))) {
    ctx.waitingKeyoff = false;
    ctx.pc++;
    return 0;
}