[fx] Draw missing image placeholders in development mode

[S1mplector]

Module: FX / Renderer

Goal:
Make missing sprite layers visually obvious during development instead of silently skipping them.

Acceptance criteria:

• Development mode draws a clear placeholder for missing image layers.
• Placeholder includes enough context to identify the missing layer when possible.
• Release mode behavior remains non-disruptive.
• Tests cover missing layer rendering path or placeholder decision logic.

Notes:
Coordinate with core missing-path warnings, but keep rendering behavior separate.