Function detect_image_support 

pub fn detect_image_support() -> ImageSupport

Detect whether the current terminal environment supports inline image rendering.

Checks are ordered: multiplexer detection first (these strip graphics escape sequences), then terminal capability, then mmdc availability.

Key distinction: we check for active session env vars, not just installed software.

• TMUX is only set inside a tmux session (not just because tmux is installed)
• STY is only set inside a screen session