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.

versions.mk

@@ -12,8 +12,8 @@ DISTRO_ID     := arclineos
 VERSION       := 0.1.0
 RELEASE_NAME  := arclines
 
-# Debian base we build on. Bookworm = Debian 12 stable.
-DEBIAN_SUITE  := bookworm
+# Debian base we build on. Trixie = Debian 13 stable.
+DEBIAN_SUITE  := trixie
 DEBIAN_MIRROR := http://deb.debian.org/debian
 SECURITY_MIRROR := http://security.debian.org/debian-security
 
@@ -21,9 +21,9 @@ SECURITY_MIRROR := http://security.debian.org/debian-security
 # stretch goal. Kept in one place so adding arm64 is a one-line change.
 ARCH          := amd64
 
-# Kernel flavour shipped by the base. Bookworm ships 6.1 LTS.
+# Kernel flavour shipped by the base. Trixie ships 6.12 LTS.
 KERNEL_PACKAGE := linux-image-amd64
-KERNEL_VERSION := 6.1
+KERNEL_VERSION := 6.12
 
 # Edition codenames (the "what do I install" flavours).
 EDITIONS      := server workstation cloud