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.

docs/building.md

@@ -73,7 +73,7 @@ Set these as environment variables or edit `versions.mk`:
 | Variable | Meaning | Default |
 |----------|---------|---------|
 | `VERSION` | release version string | `0.1.0` |
-| `DEBIAN_SUITE` | Debian base | `bookworm` |
+| `DEBIAN_SUITE` | Debian base | `trixie` |
 | `ARCH` | target architecture | `amd64` |
 | `DEBIAN_MIRROR` | mirror for the base | `http://deb.debian.org/debian` |
 | `ARCLINE_LIVE` | install live-boot into rootfs | unset (set by ISO build) |