feat(contrib): add TXT record helper with key generation

- Add contrib/make_welist_txt.sh to print the _welist-key TXT
  name, value and BIND zone line for a domain and Ed25519 key
- Accept a private/public PEM file or an existing base64 SPKI DER
- Generate private_key.pem and public_key.pem when the key argument
  is omitted, reusing an existing private key
- Document the helper in docs/10_dnssec_key_publishing.md

contrib/make_welist_txt.sh

@@ -0,0 +1,120 @@
+#!/usr/bin/env bash
+# Print the WeTime v2 DNSSEC TXT record for an executor hostname.
+set -euo pipefail
+
+usage() {
+    cat <<'EOF'
+Usage: make_welist_txt.sh <domain> [key]
+
+  <domain>  Executor hostname or URL, for example we.example.com
+  [key]     Ed25519 private/public PEM file, or the base64
+            SubjectPublicKeyInfo DER value to publish.
+
+If [key] is omitted, the script uses ./private_key.pem when it exists,
+otherwise it generates ./private_key.pem and ./public_key.pem.
+
+The record is published at _welist-key.<domain> with value:
+  v=we1; alg=ed25519; key=<base64 SPKI DER>
+EOF
+}
+
+if [ "$#" -lt 1 ] || [ "$#" -gt 2 ]; then
+    usage >&2
+    exit 2
+fi
+
+domain=$1
+key_input=${2:-}
+
+# Normalize the hostname: strip scheme, path, trailing dot, and lowercase.
+domain=${domain#http://}
+domain=${domain#https://}
+domain=${domain%%/*}
+domain=${domain%.}
+domain=$(printf '%s' "$domain" | tr '[:upper:]' '[:lower:]')
+
+# Avoid doubling the prefix if the caller already supplied it.
+domain=${domain#_welist-key.}
+
+if [ -z "$domain" ] || ! printf '%s' "$domain" | grep -Eq '^[a-z0-9.-]+$'; then
+    echo "error: invalid hostname: $domain" >&2
+    echo "       use the host only, without scheme, port, or path" >&2
+    exit 1
+fi
+
+if printf '%s' "$domain" | grep -Eq '^[0-9.]+$'; then
+    echo "error: DNSSEC key discovery requires a hostname, not an IP address" >&2
+    exit 1
+fi
+
+if [ -z "$key_input" ]; then
+    private_key="./private_key.pem"
+    public_key="./public_key.pem"
+
+    if [ -f "$private_key" ]; then
+        key_input=$private_key
+        echo "Using existing $private_key" >&2
+    else
+        if [ -e "$private_key" ] || [ -e "$public_key" ]; then
+            echo "error: $private_key or $public_key exists and will not be overwritten" >&2
+            exit 1
+        fi
+
+        if ! (umask 077; openssl genpkey -algorithm ED25519 -out "$private_key"); then
+            echo "error: cannot generate $private_key" >&2
+            exit 1
+        fi
+        if ! openssl pkey -in "$private_key" -pubout -out "$public_key"; then
+            echo "error: cannot derive $public_key" >&2
+            exit 1
+        fi
+        chmod 600 "$private_key"
+        chmod 644 "$public_key"
+
+        key_input=$private_key
+        cat >&2 <<EOF
+Generated Ed25519 key pair:
+  private key: $private_key (keep secret)
+  public key:  $public_key
+
+Set SSL_KEY_PATH and BAL_SERVER_PUB_KEY_PATH to these files.
+EOF
+    fi
+fi
+
+tmp_der=$(mktemp)
+trap 'rm -f "$tmp_der"' EXIT
+
+if [ -f "$key_input" ]; then
+    if ! openssl pkey -in "$key_input" -pubout -outform DER -out "$tmp_der" 2>/dev/null; then
+        if ! openssl pkey -pubin -in "$key_input" -outform DER -out "$tmp_der" 2>/dev/null; then
+            echo "error: cannot read an Ed25519 key from $key_input" >&2
+            exit 1
+        fi
+    fi
+else
+    printf '%s' "$key_input" | tr -d '[:space:]' \
+        | openssl base64 -d -A -out "$tmp_der" 2>/dev/null || {
+            echo "error: key is neither a PEM file nor a base64 SPKI DER value" >&2
+            exit 1
+        }
+fi
+
+der_len=$(wc -c < "$tmp_der" | tr -d ' ')
+prefix_hex=$(od -An -tx1 -N12 "$tmp_der" | tr -d ' \n')
+if [ "$der_len" -ne 44 ] || [ "$prefix_hex" != "302a300506032b6570032100" ]; then
+    echo "error: key is not a 44-byte Ed25519 SubjectPublicKeyInfo DER" >&2
+    exit 1
+fi
+
+key_b64=$(openssl base64 -A -in "$tmp_der")
+value="v=we1; alg=ed25519; key=$key_b64"
+name="_welist-key.$domain"
+
+cat <<EOF
+TXT name:  $name
+TXT value: $value
+
+BIND zone line:
+$name. 3600 IN TXT "$value"
+EOF