fix(gamescope): ship a packaged privilege path for the DM-stop takeover

Field report (Nobara, 0.19.2): entering Game Mode mid-stream left the monitor
on and the stream mirroring at the desktop's resolution. Root cause: the
DM-flavor takeover needs privilege to stop plasmalogin, the polkit rule is a
docs-only manual step nobody installs, so every managed entry silently
degraded to ATTACH — and with a physical display connected the attach path
mirrors the box's own session at its own mode.

Ship the privilege with the packages instead: a root helper
(libexec/punktfunk/pf-dm-helper, verbs stop|restore) behind its own polkit
action (io.unom.punktfunk.dm-helper, allow_any — the same mechanism Nobara's
os-session-select uses, and the helper derives the DM unit from the
display-manager.service symlink itself so callers never name a unit across
the privilege boundary). The host tries the plain system-bus verbs first
(root / operator rule still take precedence), then pkexec's the helper; the
restore paths (idle restore + in-stream desktop-switch honor) use the same
ladder so a takeover that needed the helper can always be undone by it.

Packaged in rpm/deb/arch (Arch under /usr/lib/punktfunk with the policy's
exec.path annotation rewritten; the host probes both layouts). Nix is left
out deliberately: store paths can't match the probe, NixOS keeps the manual
rule. Docs updated — the rule snippet stays as the scoped alternative.

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

packaging/debian/build-deb.sh

@@ -52,6 +52,11 @@ SHAREDIR="$STAGE/usr/share/$PKG"
 # --- file layout (matches the RPM %install) ----------------------------------
 install -Dm0755 "$BIN"                              "$STAGE/usr/bin/$PKG"
 install -Dm0644 scripts/60-punktfunk.rules         "$STAGE/usr/lib/udev/rules.d/60-punktfunk.rules"
+# Managed gamescope takeover on DM-autologin boxes: root helper + polkit action so the host can
+# stop/restore the display manager for the stream (the helper derives the DM unit itself).
+install -Dm0755 scripts/pf-dm-helper               "$STAGE/usr/libexec/punktfunk/pf-dm-helper"
+install -Dm0644 scripts/io.unom.punktfunk.dm-helper.policy \
+                                                   "$STAGE/usr/share/polkit-1/actions/io.unom.punktfunk.dm-helper.policy"
 # vhci-hcd autoload — usbip transport for the virtual Steam Deck pad (Steam only adopts USB pads).
 install -Dm0644 scripts/punktfunk-modules.conf     "$STAGE/usr/lib/modules-load.d/punktfunk.conf"
 # UDP socket-buffer tuning (32 MB) — without it the kernel clamps the host's SO_SNDBUF to ~416 KB