Add dust drift to the Plymouth shutdown logo

[ealdent]

I wanted the shutdown logo to break apart and drift away. This adds a dust-drift effect to the Plymouth theme using the existing logo and colors.

The logo crumbles from right to left, drifts upward, and fades to the background. The effect runs only during shutdown. Boot, reboot, and update modes keep the static logo. A password prompt cancels the animation and restores the logo.

Dust drift is enabled by default, with a target duration of 2.4 seconds at 30 fps. It uses at most 600 particles and prepares image crops and smaller images once. There are no new runtime dependencies or added shutdown waits. A fast poweroff can cut the animation short.

ShutdownAnimation=none keeps the static logo. ShutdownDuration accepts 0.8–5 seconds and falls back to 2.4 for invalid values.

Testing

• bash test/shell.d/plymouth-animation-test.sh passes against the native Plymouth script plugin, covering completion, disabled and invalid settings, duration limits, password prompts, boot progress, messages, and custom logos.
• bash test/shell.d/plymouth-set-test.sh passes.
• ./test/all: CLI tests pass; 228 of 229 shell-test files pass. The remaining bin-style-test.sh failure flags omarchy-remove-ai-openclaw and also reproduces on unmodified upstream eb56446.