feat: Export Audio — render resynthesis to AIFF, WAV, FLAC, Ogg, AAC/ALAC

[ideocentric]

Summary

• Adds File → Export → Export Audio… — renders unmuted/soloed partials to an audio file using the same synthesis engine as live playback ("what you hear is what you export")
• Tabbed format dialog (manila-folder style matching the existing SideTabBar) with per-format options: AIFF, WAV, FLAC, Ogg Vorbis, and AAC/ALAC (macOS only)
• AudioExporter: stateless offline renderer; creates a fresh PartialSynth, renders in 4096-sample blocks via juce::AudioFormatWriter, honours mute and solo flags at render time
• AudioExportDialog: horizontal tab strip with per-tab option panels (bit depth, compression, quality, bitrate) and a sample-rate selector; launched via DialogWindow::LaunchOptions
• PartialSynth::setProject tightened to const Project* (synth never mutates the project)
• Manual §11.7 added with four dialog screenshots; §11 intro updated to distinguish audio export from symbolic data formats; updated Export submenu screenshot

Test plan

• File → Export → Export Audio… opens the dialog defaulting to the AIFF tab
• Switching each tab updates the option panel correctly
• Export AIFF 24-bit — file plays in Finder / QuickTime, duration matches analysis
• Mute several partials, re-export — muted partials absent in output
• Test FLAC and Ogg exports — open in DAW or ffprobe
• macOS: AAC/ALAC tab present; AAC and ALAC exports play back in QuickTime
• Cancel dismisses without creating a file
• All-muted warning dialog appears when every partial is muted
• CI passes on all five targets (macOS universal, Linux x86_64/arm64, Windows x86_64/arm64)