✨ 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

test/shell.d/channel-test.sh

@@ -213,7 +213,7 @@ if grep -Eq $'^(link|refresh)\t' "$log_file"; then
   fail "dev refuses an empty region profile before linking or refreshing pacman" "$(cat "$log_file")"
 fi
 mkdir -p "$checkout/default/regions/cn/pacman"
-touch "$checkout/default/regions/cn/pacman/pacman.conf.append" "$checkout/default/regions/cn/pacman/mirrorlist.append"
+touch "$checkout/default/regions/cn/pacman/pacman.conf.append" "$checkout/default/regions/cn/pacman/mirrorlist-edge.prepend"
 chmod -x "$checkout/bin/omarchy-apply-pacman"
 if OMARCHY_TEST_REGION_FILE="$test_tmp/region" run_channel dev >"$test_tmp/region.out" 2>"$test_tmp/region.err"; then
   fail "dev refuses a checkout whose pacman helper cannot run"