fix(steamdeck): survive SteamOS A/B updates — register system tuning on the atomic-update keep list

Verified live on a SteamOS 3.8.14→3.8.16 update: the A/B partset switch
rebuilds /etc and drops everything not on Valve's keep list — the udev
rule (uhid access: virtual pads silently degrade to Xbox 360), the
vhci-hcd autoload (native Deck pad transport), and the UDP buffer sysctl
(back to 208 KB → stutter). Valve's sanctioned extension point is a
drop-in under /etc/atomic-update.conf.d/ — itself on the stock keep
list, so it self-preserves. install.sh §4 and update.sh now register
scripts/punktfunk-atomic-keep.conf there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/punktfunk-atomic-keep.conf

@@ -0,0 +1,17 @@
+# punktfunk — SteamOS atomic-update preserve list (drop-in for /etc/atomic-update.conf.d/).
+#
+# SteamOS's A/B updates rebuild /etc from scratch and carry over ONLY the files matched by
+# /usr/lib/rauc/atomic-update-keep.conf plus these operator drop-ins (the drop-in dir itself is
+# on Valve's keep list, so this file is self-preserving). Without it, every OS update silently
+# strips the host's system integration: /dev/uhid access (virtual pads degrade to Xbox 360),
+# the vhci-hcd autoload (no native Steam Deck pad), and the UDP buffer sizing (stream stutter).
+# Same wildcard rules as the stock list: '*' matches within a path segment, '**' across.
+
+## Virtual-gamepad device access (uinput/uhid/vhci for the input group)
+/etc/udev/rules.d/60-punktfunk.rules
+
+## vhci-hcd autoload (usbip transport for the native Steam Deck pad)
+/etc/modules-load.d/punktfunk.conf
+
+## UDP socket buffer sizing (32 MB, matches the deb's sysctl drop-in)
+/etc/sysctl.d/99-punktfunk-net.conf