Add OOP EQ knob controls (Low/Mid/High) using existing knob asset

[Copilot]

This PR adds three new EQ controls (Low, Mid, High) alongside the existing volume knob, reusing the same knob image and integrating drag-based interaction in the main player UI. The implementation introduces explicit OOP control primitives so knob behavior is clear, reusable, and isolated from rendering/event plumbing.

• OOP control model

  • Added eq_controls.py with:
    • KnobControl: pointer hit-testing, angle/value mapping, render angle, label/percent helpers.
    • EqualizerControls: knob collection management + drag lifecycle (begin_drag, drag_to, end_drag).
  • Centralized knob math and state transitions out of the event loop.

• UI + interaction integration

  • Replaced single-volume ad hoc state in main.py with four knob instances:
    • volume, low, mid, high.
  • Unified mouse drag handling through EqualizerControls.
  • Rendered all four knobs with the existing volume.png and per-knob labels/percent values.

• Audio behavior updates

  • Volume continues to use the existing taper mapping.
  • Low/Mid/High now influence live output via a bounded presence gain derived from EQ knob positions, so all added knobs are functional at runtime.

• Docs and focused coverage

  • README updated to describe volume + EQ knob behavior.
  • Added focused unit tests for knob mapping, drag flow, and invalid active-knob safety handling.

eq_controls = EqualizerControls([
    KnobControl(name="volume", label="Vol", center=(410, 582), value=0.5),
    KnobControl(name="low",    label="Low", center=(470, 582), value=0.5),
    KnobControl(name="mid",    label="Mid", center=(530, 582), value=0.5),
    KnobControl(name="high",   label="High", center=(590, 582), value=0.5),
])

changed_knob = eq_controls.drag_to(mouse_pos)
if changed_knob in {"volume", "low", "mid", "high"}:
    _apply_volume_and_eq()