iso: add bos-rescue, first-boot probe, optional Calamares refresh

Live-ISO bos-rescue finds the installed btrfs @ and ESP, then offers
arch-chroot and/or the same GRUB NVRAM + --removable sequence as
post-install.sh. Recovery is grub-btrfs or this reinstall — GRUB pins
rootflags=subvol=@.

bos-first-boot runs once after the first graphical login: NVIDIA offer
file + notify (no driver install), VM-without-GL notify, HiDPI hint
file (never rewrites monitors.json).

Re-enable the Calamares packages module as a refresh-only step with
skip_if_no_internet and ignore_update_db_error so offline installs
cannot abort on pacman -Sy.

iso/airootfs/etc/calamares/modules/packages.conf

@@ -1,10 +1,24 @@
 ---
+# Optional online pacman refresh. The previous packages step used
+# update_db:true with no skip/ignore, so `pacman -Sy` aborted offline
+# installs (the case bos-netcheck exists for). skip_if_no_internet
+# skips the whole module when Calamares sees no network;
+# ignore_update_db_error keeps a flake-mirror -Sy from failing the
+# install. update_system stays false — this is not a -Syu.
+#
+# try_install is empty: pipewire-pulse / pipewire-alsa already come
+# from packages.x86_64 via unpackfs. No extra packages (and no
+# nvidia) are pulled here.
 backend: pacman
 
-options:
-  - update_db: true
+skip_if_no_internet: true
+update_db: true
+ignore_update_db_error: true
+update_system: false
 
-operations:
-  - try_install:
-    - pipewire-pulse
-    - pipewire-alsa
+pacman:
+  num_retries: 1
+  disable_download_timeout: false
+  needed_only: true
+
+operations: []