max: fix crash on DSP start with fixed/wider output buses

[jcelerier]

Bug

A Max audio external built from an object with a fixed output bus (e.g. fixed_audio_bus<double, 2>) crashes the instant audio (DSP) starts when fed a mono source.

Root cause

audio_processor's multichanneloutputs handler reported the runtime input channel count:

static constexpr auto outputcount = +[](instance* x, long index) -> long {
    // FIXME compute it better
    return x->m_runtime_input_count;
};

But dsp() allocates — and the object writes — max(output_channels, input_chans) output channels. With a mono cycle~ → the effect, Max was told to allocate 1 output channel, while the object's operator() unconditionally writes outputs.audio.channel(1, N) too → out-of-bounds write in perform() → hard crash on the first DSP tick.

Fix

Report the same count dsp() uses:

const long ins = std::max<long>(instance::input_channels, x->m_runtime_input_count);
return std::max<long>(instance::output_channels, ins);

A mono source into a fixed-stereo effect now gets 2 output channels allocated, matching what the object writes.

Verification

Built the oscr_Distortion external (Tutorial/Distortion, fixed_audio_bus<double, 2> output) and ran it in Max 8: cycle~ 220 → oscr_Distortion → ezdac~ with DSP auto-started via loadbang → [; dsp 1(. Before: crash on DSP start. After: runs cleanly for 30s+, empty Max console, no crash dialog.

Found while GUI-verifying the auto-generated help patches (unrelated PR #126); this is a pre-existing Max-binding bug.

🤖 Generated with Claude Code