chore: base Arcline OS on Debian trixie (current stable)

Bookworm became oldstable in July 2026 (standard support ended
2026-07-11). A new distro should be built on the current stable, which
is now Trixie (13.6): newer kernel (6.12 LTS vs 6.1, whose LTS ends Dec
2026), newer systemd/containers/toolchains, KDE 6 for the workstation
edition, and a year of stability behind it (full support until
2028-08-09).

- versions.mk / common.sh: DEBIAN_SUITE bookworm -> trixie, kernel 6.1 -> 6.12
- package list header comments: bookworm -> trixie
- GitLab CI: debian:bookworm -> debian:trixie build images
- docs (architecture, building): base references updated

All packages used across the three editions exist in Trixie unchanged.

scripts/common.sh

@@ -13,12 +13,12 @@ set -euo pipefail
 : "${DISTRO_ID:=arclineos}"
 : "${VERSION:=0.1.0}"
 : "${RELEASE_NAME:=arclines}"
-: "${DEBIAN_SUITE:=bookworm}"
+: "${DEBIAN_SUITE:=trixie}"
 : "${DEBIAN_MIRROR:=http://deb.debian.org/debian}"
 : "${SECURITY_MIRROR:=http://security.debian.org/debian-security}"
 : "${ARCH:=amd64}"
 : "${KERNEL_PACKAGE:=linux-image-amd64}"
-: "${KERNEL_VERSION:=6.1}"
+: "${KERNEL_VERSION:=6.12}"
 
 # Toolchain policy for image builds:
 #   auto    (default) install the Arcline tools if build/debs/*.deb exist,