linea: add engine API healthcheck script for geth mainnet

Shell probe signs a JWT from /jwtsecret and calls engine_exchangeCapabilities
on :8551 so compose can gate Maru startup on EL engine readiness.

linea/scripts/engine-healthcheck.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+apk add --no-cache openssl wget >/dev/null 2>&1 || true
+SECRET=$(tr -d '\n' </jwtsecret | sed 's/^0x//')
+H=$(printf '{"alg":"HS256","typ":"JWT"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
+P=$(printf '{"iat":%s}' "$(date +%s)" | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
+SIG=$(printf '%s' "$H.$P" | openssl dgst -sha256 -mac HMAC -macopt hexkey:"$SECRET" -binary | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
+wget -q -O /dev/null \
+  --header="Content-Type: application/json" \
+  --header="Authorization: Bearer $H.$P.$SIG" \
+  --post-data='{"jsonrpc":"2.0","method":"engine_exchangeCapabilities","params":[],"id":1}' \
+  http://127.0.0.1:8551