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.

ci/.gitlab-ci.yml

@@ -4,7 +4,7 @@
 # Mirrors the local build flow (scripts/ + Makefile) in CI:
 #   validate → build (matrix over editions) → test → publish
 #
-# The build image is Debian bookworm with the host deps installed, matching
+# The build image is Debian trixie with the host deps installed, matching
 # what scripts/check-host-deps.sh expects locally.
 # ─────────────────────────────────────────────────────────────────────────────
 stages:
@@ -20,7 +20,7 @@ variables:
 # ── validate ────────────────────────────────────────────────────────────────
 validate:
   stage: validate
-  image: debian:bookworm
+  image: debian:trixie
   script:
     - apt-get update -qq && apt-get install -y -qq make bash
     - make check
@@ -28,7 +28,7 @@ validate:
 # ── build (one job per edition) ─────────────────────────────────────────────
 .build:
   stage: build
-  image: debian:bookworm
+  image: debian:trixie
   before_script:
     - apt-get update -qq
     - apt-get install -y -qq debootstrap squashfs-tools grub2-common xorriso cpio curl git make bash gdisk parted rsync dosfstools qemu-utils unzip dpkg openssl sbsigntool
@@ -63,7 +63,7 @@ build-cloud:
 # ── test ────────────────────────────────────────────────────────────────────
 test:
   stage: test
-  image: debian:bookworm
+  image: debian:trixie
   needs: [build-server, build-workstation, build-cloud]
   before_script:
     - apt-get update -qq && apt-get install -y -qq bash make xz-utils