Precise user-idle detection for retreat (ext-idle-notify)

Context

The retreat behavior (pet ambles to a corner and sleeps) has two intended triggers:

• A — rare cadence (~every 30 min of runtime): implemented / planned as the reliable path.
• B — user inactivity (~5 min of no input → sleep until the user returns): stubbed for now.

Why B is stubbed

goob is a click-through overlay with mouse passthrough clipped to the pet's rect, so the app never sees the global cursor when it's off the pet — we can't measure idleness from cursor movement. And on the dev machine (KDE Plasma / kwin_wayland), the easy system idle sources don't work on Wayland:

• org.freedesktop.ScreenSaver.GetSessionIdleTime → NotSupported on Wayland (X11-only).
• logind IdleHint stays no / only flips at the coarse system idle timeout.
• No swayidle/kidletime present.

The proper cross-compositor signal is the ext-idle-notify-v1 Wayland protocol (KWin supports it), which needs a native Wayland client — a small GDExtension or an external helper binary invoked from Godot.

Task

Implement Trigger B via a real idle source:

• ext-idle-notify-v1 helper (GDExtension or external process that emits idle/resume events), or a best-effort logind IdleHint poll where it works.
• Wire it into the stubbed retreat idle hook in the behavior engine.
• Feature-detect and degrade gracefully (cadence-only) when no idle source is available.

Until then, retreat runs on the cadence trigger only.