✨ feat(region): put the china channel mirrors first

- ✨ omarchy now runs per-channel china replicas, so they should lead instead of trailing as a fallback
- ♻️ prepend per-channel fragments so each channel's own mirror stays the fallback
- 🔥 drop the ustc arch mirror, which was never a stable or rc snapshot replica

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QuATgjU4mnqpqSLAKMQ14g

default/regions/cn/pacman/mirrorlist-edge.prepend

@@ -0,0 +1 @@
+Server = https://mirror.omarchy.cn/$repo/os/$arch

default/regions/cn/pacman/mirrorlist-rc.prepend

@@ -0,0 +1 @@
+Server = https://rc-mirror.omarchy.cn/$repo/os/$arch

default/regions/cn/pacman/mirrorlist-stable.prepend

@@ -0,0 +1 @@
+Server = https://stable-mirror.omarchy.cn/$repo/os/$arch

default/regions/cn/pacman/mirrorlist.append

@@ -1 +0,0 @@
-Server = https://mirrors.ustc.edu.cn/archlinux/$repo/os/$arch