fix(common): resolve_a must not abort under set -e on NXDOMAIN

dig | grep | head pipeline returned 1 when grep found no IPs, which
under `set -euo pipefail` killed 05-preflight.sh inside `a=$(resolve_a ...)`
before the empty-string branch could handle it. Wrap the pipeline so the
function always exits 0 with empty stdout on failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: qfiber

lib/common.sh

@@ -217,8 +217,9 @@ detect_public_ip() {
 # Resolve A record for $1 against a public resolver (1.1.1.1).
 # Echoes IP or empty.
 resolve_a() {
-    local host="$1"
-    dig +short +time=2 +tries=1 A "${host}" @1.1.1.1 2>/dev/null | grep -E '^[0-9]+\.' | head -1
+    local host="$1" out
+    out=$(dig +short +time=2 +tries=1 A "${host}" @1.1.1.1 2>/dev/null || true)
+    printf '%s\n' "${out}" | grep -E '^[0-9]+\.' | head -1 || true
 }
 
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