Merge reworked #13690: timezone-derived region and channel mirror prepends

Drop the .cn servers.sed twins, the USTC Arch fallback and the closed
#14399 timezone preselection. region.sh follows the installer's marker.

test/shell.d/setup-form-test.sh

@@ -61,7 +61,7 @@ fi
 
 trap 'if [[ ${FUNCNAME[0]:-} == "$PROMPT_FN" ]]; then printf "returned\n" >>"$MARKER"; fi' RETURN
 
-"$PROMPT_FN" ${PROMPT_ARG:+"$PROMPT_ARG"}
+"$PROMPT_FN"
 
 printf 'region=%s\n' "${region:-}"
 printf 'keyboard=%s\n' "${keyboard:-}"
@@ -274,19 +274,6 @@ assert_status 0 "timezone prompt survives a failed geo guess"
 [[ $(head -n 1 "$GUM_ARGS") == filter* ]] || fail "timezone prompt filters when there is no geo guess"
 pass "timezone prompt falls back to filtering when the geo guess fails"
 
-# A region's default stands in for the missing guess, so an offline China
-# install opens on Asia/Shanghai instead of the top of the list
-PROMPT_ARG=Asia/Shanghai run_prompt omarchy_prompt_timezone "0:Asia/Shanghai"
-assert_status 0 "timezone prompt accepts the caller's fallback"
-[[ $(field timezone) == "Asia/Shanghai" ]] || fail "timezone prompt keeps the preselected fallback"
-grep -qF -- '--selected Asia/Shanghai' "$GUM_ARGS" || fail "timezone prompt preselects the caller's fallback"
-[[ $(head -n 1 "$GUM_ARGS") == choose* ]] || fail "timezone prompt chooses from the list when it has a fallback"
-pass "timezone prompt preselects the caller's fallback when the geo guess fails"
-
-TZ_GUESS=Europe/Copenhagen PROMPT_ARG=Asia/Shanghai run_prompt omarchy_prompt_timezone "0:Europe/Copenhagen"
-grep -qF -- '--selected Europe/Copenhagen' "$GUM_ARGS" || fail "a geo guess outranks the caller's fallback"
-pass "timezone prompt prefers the geo guess over the caller's fallback"
-
 run_prompt omarchy_prompt_timezone "0:"
 assert_status 0 "timezone prompt accepts an empty selection"
 [[ $(field timezone) == "UTC" ]] || fail "timezone prompt falls back to UTC"