[fx] Expose layered draw count in performance HUD

[S1mplector]

Module: FX / Renderer

Goal:
Expose a renderer metric that helps diagnose expensive layered character scenes.

Acceptance criteria:

• Performance HUD can show total layered draw calls for the current frame.
• Metric distinguishes character layer draws from other sprite/background draws where practical.
• Metric is low overhead and can be disabled with existing HUD controls.
• Documentation briefly explains how to interpret the number.

Notes:
This should help creators understand why a scene is heavy.