Phase 33: add a flat modern DAW workspace and track creation workflow

docs/WORKSPACE.md

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+# Flat DAW workspace
+
+Phase 33 presents the existing xtc editors in a consistent flat workspace. It uses
+FLTK vector drawing and system fonts: no images, shadows, bevels, 3D rendering,
+external theme packages or new runtime dependencies.
+
+The default window is 1280 × 800; the supported minimum is 1220 × 720. The main
+editor grows with the window. Use the scrollbars for longer arrangements and
+larger track counts. The sidebar stays available across Sequencer, Mixer and
+Modular Graph views. Ctrl+1/2/3 switches those views; Ctrl+4 opens the piano roll.
+
+## Create and arrange
+
+- **New MIDI track** (Ctrl+Shift+M) creates a named track and a one-bar MIDI clip
+  at the cursor. The new clip and mixer track are selected together. **Edit MIDI
+  notes** opens its piano roll. Creation is one undo step.
+- **New audio track** (Ctrl+Shift+A) creates an empty named track. **Import WAV**
+  imports into the selected track, or creates one when none is selected.
+- **Load instrument plugin** opens the instrument browser for the selected
+  sequencer track. Unassigned MIDI tracks retain the built-in sine instrument.
+- Existing Add track/region/MIDI clip, duplication, trimming, split, rename and
+  delete commands remain available. F2 renames the selection. Drag clips to move
+  them and their edges to trim them; click the ruler to seek while stopped.
+
+Creating a track uses candidate preparation, so module, capacity or allocation
+failures retain the current project. As with other project replacements, the
+engine stops and the transport returns to the beginning. Track creation is
+unavailable while recording. No track type restriction is added to the model:
+ordinary tracks still accept the existing audio and MIDI workflows.
+
+Track headers show a stable color, name, order and mute/solo state. Clip colors
+follow track identity through reordering. MIDI previews show real note starts,
+durations and relative pitches; waveforms continue to show loaded audio data.
+The selected clip has a light outline. A teal playhead and bar grid make musical
+time visible. The transport display reads **bar : beat : tick** (4/4, 960 ticks
+per beat); BPM follows the project tempo. These displays are read-only.
+
+The sidebar inspector follows the sequencer selection, showing track name,
+gain/pan, mute/solo and assigned instrument. Use the Mixer for controls, routing,
+meters and automation; the sidebar is an overview, not another mixer. Status and
+errors remain visible below the editor. Existing engine connection, transport,
+recording and keyboard safety behavior is retained.
+
+## Example and verification
+
+Open `examples/projects/modern-session.xtc` for an original four-track, four-bar
+MIDI arrangement with 16 clips and 128 notes. It needs no media or plugins. It is
+also installed with the project examples. Connect JACK playback ports before
+listening. Work on a copy if the installed examples are read-only.
+
+GUI tests exercise sidebar creation, selected clip/editor access, undo/redo,
+failed preparation, save/reopen and layout at 1220 × 720 and 1600 × 1000. The
+running example was also inspected at 1280 × 800. Existing GUI plugin, live JACK
+and workflow tests cover the retained commands. A modern presentation does not
+add commercial-DAW features: detachable/resizable docks, mixer channel strips,
+automatic plugin scanning, custom plugin editors and new audio/MIDI features
+remain outside this phase. Existing plugin and engine limits still apply.