Complete archiso/Calamares profile scaffold (iso/)

- Calamares modules: unpackfs (clone), partition/mount/fstab (btrfs subvols),
  bootloader (systemd-boot + quiet splash), packages (strip live), services
  (greetd), shellprocess (snapper, flathub, strip-live), branding (Fuji)
- bootstrap-profile.sh: releng base + linux-zen swap + overlay our deltas
- BUILD.md: build steps, decisions, and honest open items (needs build-test)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/overrides/airootfs/etc/calamares/modules/mount.conf

@@ -0,0 +1,37 @@
+---
+# Snapper-friendly Btrfs subvolume layout (@ root + @snapshots at /.snapshots).
+extraMounts:
+- device: proc
+  fs: proc
+  mountPoint: /proc
+- device: sys
+  fs: sysfs
+  mountPoint: /sys
+- device: /dev
+  mountPoint: /dev
+  options: bind
+- device: tmpfs
+  fs: tmpfs
+  mountPoint: /run
+- device: /run/udev
+  mountPoint: /run/udev
+  options: bind
+
+mountOptions:
+- mountPoint: /
+  options: defaults,noatime
+- mountPoint: "*"
+  filesystem: btrfs
+  options: defaults,noatime,compress=zstd
+
+btrfsSubvolumes:
+- mountPoint: /
+  subvolume: /@
+- mountPoint: /home
+  subvolume: /@home
+- mountPoint: /var/cache
+  subvolume: /@cache
+- mountPoint: /var/log
+  subvolume: /@log
+- mountPoint: /.snapshots
+  subvolume: /@snapshots