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/branding/tanin/branding.desc

@@ -0,0 +1,24 @@
+---
+componentName: tanin
+welcomeStyleCalamares: true
+windowExpanding: normal
+
+strings:
+    productName:         TANINUX
+    shortProductName:    TANINUX
+    version:             camel
+    shortVersion:        camel
+    versionedName:       TANINUX (camel)
+    shortVersionedName:  TANINUX
+    bootloaderEntryName:  TANINUX
+    productUrl:          https://taninux.kgva.ch
+    supportUrl:          https://git.openbureau.ch/karim/taninux
+
+# Fuji palette for the installer sidebar (matches the desktop).
+style:
+    sidebarBackground:    "#0f0f0f"
+    sidebarText:          "#f0f0f0"
+    sidebarTextSelect:    "#a39ec4"
+    sidebarTextHighlight: "#a39ec4"
+
+# TODO: add productLogo (logo.png) + a slideshow (show.qml) for polish.