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/steamdeck/install.sh

@@ -274,6 +274,14 @@ if [ "$SUDO_OK" = 1 ]; then
         NEED_RELOGIN=1
         warn "added $USER to the 'input' group (applies on next login)"
     fi
+    # SteamOS A/B updates rebuild /etc and DROP everything not on Valve's keep list — verified
+    # live: an OS update stripped the udev rule + vhci autoload + UDP sysctl (gamepads silently
+    # degrade to Xbox 360, buffers back to 208 KB). The sanctioned fix is a preserve drop-in in
+    # /etc/atomic-update.conf.d/ (itself on the stock keep list, so it self-preserves).
+    if [ -f "$SRC/scripts/punktfunk-atomic-keep.conf" ]; then
+        sudo install -Dm644 "$SRC/scripts/punktfunk-atomic-keep.conf" /etc/atomic-update.conf.d/punktfunk.conf
+        ok "system tuning registered to survive SteamOS updates (atomic-update.conf.d)"
+    fi
 else
     warn "no usable sudo — SKIPPED system tuning. Gamepad passthrough + clean streaming need root (udev"
     warn "rule, 'input' group, vhci-hcd, UDP buffers) — there is no user-space way to do these."