Add Arch packaging and harden the Steam webhelper lifecycle

[m1ngsama]

Summary

• add a deckless-git Arch package entry point with PKGBUILD, .SRCINFO, and /usr/share/deckless install wrappers
• document the Arch flow and include a real-machine Big Picture screenshot in the README
• split the Steam wrapper handling into sync, heal, and detached cleanup helpers so updater verification stays clean while the proxied GPU-enabled webhelper path still lands reliably

Testing

• bash -n bin/deckless-steam bin/deckless-sync-webhelper-wrapper bin/deckless-webhelper-heal bin/deckless-webhelper-cleanup bin/deckless-bigpicture bin/deckless-i3-bigpicture-bridge install.sh uninstall.sh
• shellcheck bin/deckless-steam bin/deckless-sync-webhelper-wrapper bin/deckless-webhelper-heal bin/deckless-webhelper-cleanup bin/deckless-bigpicture bin/deckless-i3-bigpicture-bridge install.sh uninstall.sh
• env HOME=/tmp/deckless-test-home XDG_CONFIG_HOME=/tmp/deckless-test-home/.config XDG_DATA_HOME=/tmp/deckless-test-home/.local/share ./install.sh
• env HOME=/tmp/deckless-test-home XDG_CONFIG_HOME=/tmp/deckless-test-home/.config XDG_DATA_HOME=/tmp/deckless-test-home/.local/share ./uninstall.sh
• real-machine validation on Arch Linux + X11 + i3 + NVIDIA 595.58.3 + 240 Hz: Steam updater verification completed without wrapper mismatch loops, the top-level steamwebhelper relaunched with --proxy-server=... and without --disable-gpu*, webhelper_gpu.txt reported ANGLE_OPENGL, gpu_compositing: enabled, opengl: enabled_on, video_decode: enabled, and the detached cleanup helper restored the official 623-byte wrapper after Steam exited

Closes #5