Add freeplay recording with in-app preview and MIDI export

[rumycoding]

Intro

Hi! I’ve been using Neothesia and really enjoy it. To support personal practice, I added a freeplay recording feature that captures live MIDI input, converts the take into a playable song, and allows saving it locally as a MIDI file.

Summary

This PR introduces an in-app recording flow: when recording stops, the app builds a preview of the take directly—no external reload required. It also adds a preview UI with record/save/play/pause, seeking, progress tracking, measure markers, and waterfall/note rendering, all integrated with existing scene components. Supporting changes include in-memory SMF construction and safeguards around playback math to handle zero-length or very short recordings gracefully.

File changed

• [neothesia/src/scene/freeplay/mod.rs]: adds freeplay recording, in-app preview playback, progress/seek UI, and MIDI export.
• [midi-file/src/file.rs]: adds in-memory MIDI loading so recorded takes can be previewed without writing a temporary file.
• [neothesia/src/scene/playing_scene/midi_player.rs]: supports configurable lead-in timing and safer seek/progress handling for preview playback.
• [midi-file/src/playback.rs]: guards playback progress calculations when a recording has zero length.
• [neothesia/src/scene/playing_scene/keyboard.rs]: lets freeplay preview apply generated song configuration to the keyboard UI.
• [neothesia/src/scene/playing_scene/mod.rs]: exposes the MIDI player module for reuse in freeplay preview.