Repaint: local audio region regeneration

[thedavidweng]

Problem

No way to regenerate a portion of an existing audio file. Users must regenerate from scratch to iterate on a section.

Scope

• Waveform drag-select to define a region for regeneration
• ACE-Step /edit endpoint integration for local audio region replacement
• UI for repaint parameters (region, new prompt, strength)

Acceptance criteria

• User can select a region on the waveform
• Selected region can be regenerated with a new prompt via /edit
• Repainted audio replaces or appends to the original file
• CLI supports repaint via openloop run --repaint <file> --start <s> --end <s>