Step Sequencer

Summary

This is a major feature release. It adds a complete acid step sequencer ported
from Aciduino V2, two new DSP contributions (a Pirkle diode ladder filter and
an SH-101-style LFO), a brand-new midilabv2 GUI theme, plus standalone builds
and a Docker cross-compile builder for Linux.

Features

1. Aciduino V2 Acid Step Sequencer (major)

Full JUCE port of the Aciduino V2 engine
(engine_303.cpp + harmonizer.cpp) into src/sequencer/AcidSequencer303.h,
ticked sample-accurately from processBlock().

• PPQN-agnostic timing derived from the host/internal resolution; authentic
  303 note length (50% gate), slide overhang, and tie-extension ratios
• Three sync modes: Internal, Host (AudioPlayHead), MIDI Clock
• Transport-aware start via host play / MIDI Start or note-triggered
• Rec mode: record notes live from the plugin keyboard or MIDI input via the
  on-screen SeqKeyboard (octave control via mouse wheel, notes only in rec —
  accent/slide/tie are not overwritten)
• Generative controls: harmony, sequence shift, sequence length
• Randomize and clear-track; slide/tie bridge correctly from the last step back
  to the first
• UI command channel (play/stop/generate/clear) consumed on the audio thread to
  avoid race conditions; CPU usage tuned for the audio thread
• New shared GUI components: MenuPage.h, SeqKeyboard.h, step selector,
  switch/step-button components

Contributors

• midilab — port and UI/UX implementation (original Aciduino engine by
  Romulo Silva, MIT License)

2. Diode Ladder Filter (major)

New dfl_DiodeLadderFilter (DSP from the Pirkle diode ladder) integrated into
the Open303 voice with selectable filter type plus a "Filter Model" editor
parameter.

• Filter types: TB-303 TeeBee, Diode Ladder (4-pole, 24 dB/oct),
  Diode Octave (1st pole one octave above, ~18 dB/oct)
• SATURATION drive into the diode saturator (~4.5 dB default) and passband
  (bass) compensation to offset high-resonance thinning
• Wired into the new menu page and mods schema

Contributors

• David Löwenfels — Pirkle diode ladder filter implementations
• midilab — integration into the GUI (MenuPage), mods schema, and parameter
  plumbing

3. LFO Modulation

SH-101-inspired LFO with sine/triangle/sawtooth/square/sample-&-hold/noise
waveforms and square pulse-width control.

• Refactored to a single modulation destination at a time with a global depth
  (fewer interface knobs), plus volume as a destination for vibrato-style
  effects
• Pitch modulation restored, square pulse width now driven by the symmetry
  parameter

Contributors

• David Löwenfels — LFO implementation and pulse-width modulation
• midilab — destination/depth refactor, parameter-map fixes, volume-pitch
  destinations wording

4. midilabv2 Theme (major)

New full-fidelity GUI theme replacing the old panel, reusing the whole feature
set (sequencer UI, menu page, mods). Includes new resources: photorealistic
screws, knobs, LCD fonts, sequencer step selector artwork, and a redesigned
drop-down menu.

Contributors

• midilab

5. Standalone Build & Docker Cross-Compile

• Added Standalone to the JUCE plugin formats with a new app icon
• Docker builder supporting native and cross-compiled x86_64 / arm64
  Linux releases (--build-arg RELEASE_ARCH=X86_64|ARM64), documented in the
  README

Contributors

• midilab

What's Changed

• feat: LFO modulation (and PWM) - inspired by SH-101 by @dfl in #34
• Feature/aciduino step sequencer by @midilab in #54
• feat: add Pirkle diode ladder filter with filter type selection by @dfl in #33
• feat: new theme by @midilab

New Contributors

• @dfl made their first contribution in #34

Full Changelog: v0.12.3...v0.13.0