feat(gamescope/hdr): HDR is attempted by default and every install path ships the patched gamescope

Field report (RX 5700 / SteamOS): 10-bit HEVC "unsupported" — but the GPU was
never the blocker (RADV probed Main10 fine on that box). The session stayed SDR
because stock gamescope offers no 10-bit PQ capture formats and nothing shipped
our pipewire-hdr build outside the Bazzite sysext, while PUNKTFUNK_GAMESCOPE_HDR
additionally defaulted off everywhere. Close all three gaps:

- PUNKTFUNK_GAMESCOPE_HDR now defaults ON (explicit-off grammar, the flip the
  config comment planned post-canary). Safe by construction: the +pfhdr banner
  probe, managed-spawn term, and client 10-bit cap keep stock-gamescope boxes on
  the exact 8-bit path. The nix module's gamescopeHdr (default true now) only
  controls whether the patched binary is on PATH; it no longer sets the env.
- SteamOS: new scripts/steamdeck/build-gamescope.sh builds the HDR gamescope in
  the same pf2 trixie distrobox (apt dep list validated by a full build in a
  debian:trixie container), installs ~/.local/bin/punktfunk-gamescope, and
  maintains PUNKTFUNK_GAMESCOPE_BIN in host.env ONLY while the binary passes its
  on-glass --version/+pfhdr check — a stale absolute override would break
  session spawning, not just HDR. Wired into install.sh (§3b), update.sh
  (retrofit), and rebuild-check.sh (ldd-probes it like the host binary).
- Arch: build-sysext.sh gains a --gamescope fold-in (banner-verified by
  execution, host-image-only), and punktfunk-host optdepends the companion.
  Deliberately NO CI-published host sysext: per the 2026-07-22 packaging
  verdict a prebuilt SteamOS host breaks on A/B soname bumps (and /var is
  per-partition-set), so arch.yml records that instead — SteamOS hosts use the
  distrobox flow above.

Also fixes a latent bug that made build-sysext.sh fail every invocation as
documented: a literal `}` inside `${1:?usage …{host,client}…}` terminates the
expansion early and corrupts $PKG (caught by the new synthetic-package test,
which also covers the fold-in, the unmarked-gamescope refusal, and the
client-image guard).

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

scripts/steamdeck/update.sh

@@ -52,6 +52,12 @@ sed 's|^ExecStart=.*|ExecStart=%h/.local/bin/punktfunk-scripting|' \
 systemctl --user daemon-reload
 ok "plugin runner rebuilt (opt-in service: systemctl --user enable --now punktfunk-scripting)"
 
+# HDR gamescope (punktfunk-gamescope): rebuild when the packaging tree changed or the installed
+# binary stopped working — also RETROFITS it onto older installs that predate it (fast no-op
+# otherwise). Best-effort; on failure the host streams SDR (see build-gamescope.sh).
+log "HDR gamescope (punktfunk-gamescope)"
+PUNKTFUNK_SRC="$SRC" PUNKTFUNK_BOX="$BOX" bash "$SRC/scripts/steamdeck/build-gamescope.sh"
+
 # Retrofit the post-OS-update rebuild check (install.sh §5) onto older installs: probes the host
 # binary with ldd at session start and re-runs this script when a SteamOS update broke its links.
 if [ ! -f "$HOME/.config/systemd/user/punktfunk-rebuild-check.service" ]; then