Align the China region to the official cn profile

The region value is cn, the China files live under default/regions/cn/, and
omarchy-apply-pacman renders pacman for every caller: the .cn twins of the
channel servers first, then .org, USTC and ArchLinuxCN. Switching to China
installs and trusts the ArchLinuxCN keyring. A migration renames china to cn
and repoints the drop-in links.

test/shell.d/menu-test.sh

@@ -302,7 +302,7 @@ assert(
   regionEntries.map(item => item.label).join('\0') === 'Global\0China'
     && regionEntries.every(item => item.checked.includes(`== \"${item.label}\"`) && !item.when)
     && defaultById['setup.region.global'].action.includes('omarchy-region global')
-    && defaultById['setup.region.china'].action.includes('omarchy-region china'),
+    && defaultById['setup.region.china'].action.includes('omarchy-region cn'),
   'menu offers Global and China under Setup > Region'
 )
 // Software you already have keeps its place in Install, dimmed rather than