Enable full MIDI note support (0-127 instead of 12-108) in VST sequencer

[eakwarren]

This PR enables full MIDI note support in the VST Sequencer. It doesn't touch fluidsynth or MuseSampler, but similar noteIndex and noteTuning methods in fluidsequencer.cpp and musesamplersequencer.cpp could be ported in subsequent PRs.

While ideally this should be fixed in pitchutils.h as described in #16657, this small PR unlocks full VST instrument and keyswitch support for the short term and incurs little tech debt as only two methods are changed.

• Allow raw MIDI 0..127 through the VST sequencer; remove the old C0..C8 clamp.
• Introduce MIDI_MIN_NOTE=0 and MIDI_MAX_NOTE=127; delete MAX_SUPPORTED_PITCH_LEVEL.
• noteIndex():
  • Primary: compute idx relative to C0 (MIDI 12) and std::clamp to 0..127.
  • Fallback: for wrapped “below C0” pitch levels, map to keyswitch zone 0..11 using a modulo phase fix ((stepsRounded + 5) % 12) to eliminate 11→6 offset.
• noteTuning():
  • Derive tuning versus the effective MIDI idx returned by noteIndex().
  • Zero tuning for keyswitch notes (idx < 12) to avoid large wrapped cent values.

Tested with Vital Synth, Kontakt 8, Sine Player, Synchron Player, Spitfire Player, MuseSounds, MS Basic.

Resolves: #16657

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[eakwarren]

@cbjeukendrup @avvvvve What do you think? There may not be much value in porting to fluidsynth or MuseSampler if their instruments don't use the full 0-127 midi note range. But for consistency, I'm happy to do so.

[avvvvve]

I'll let @cbjeukendrup answer the technical bits, but it sounds like we will certainly need a fix like this if we want to get keyswitches working with automation down the line!

[cbjeukendrup]

I'm afraid I don't understand why this fallback logic is necessary. I would expect that MIDI note 0 is represented as -12 * mpe::PITCH_LEVEL_STEP in pitch_level_t, and if that's the case, everything should just work. But apparently, that's not the case and I've yet to find out why. (pitch_level_t is signed, so that's also not the source of trouble)

[cbjeukendrup]

UPDATE: I think I've found the problem. pitch_level_t is indeed a signed type. But octave_t not. Fixing that seems to fix all trouble.

[eakwarren]

Sweet! Looking forward to testing it in nightly. @cbjeukendrup want me to close this PR?

[cbjeukendrup]

You could test my PR: #31809 and if that works, you could indeed close this one.