sync-status: dRPC-homogeneous block-lag status + fix never-used reference fallbacks

Match the dRPC gateway's per-chain "how many blocks behind is ok" model instead of a
fixed 2s/5s timestamp tolerance:
- check-health.sh: compare the reference head vs local head by BLOCK NUMBER and classify
  with the chain's dRPC lag thresholds (LAGGING_LAG/SYNCING_LAG, in blocks, from
  chains.yaml). dRPC uses the two thresholds inconsistently across chains (sometimes
  lagging<syncing, sometimes the reverse) so the smaller is the online boundary and the
  larger the syncing/drop boundary. Defaults 2/6 when a chain has no thresholds.
- multicurl.sh: also skip responses with result:null (a lagging endpoint lacking the
  requested block) so the fallback reference URLs are actually tried. Previously the first
  endpoint's {"result":null} was accepted as success -> fallbacks never ran, and the null
  reference hash made check-health report false "forked" (the online/forked flapping).
- sync-status.sh: resolve the lag thresholds (by drpc slug or chain id) and export
  LAGGING_LAG/SYNCING_LAG.
- reference-rpc-endpoint.sh: add --lags and --block-time-ms lookups.
- reference-rpc-endpoint.json: regenerated with per-chain block_time_ms + lagging_lag +
  syncing_lag (additive).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sync-status.sh

@@ -158,6 +158,24 @@ for path in $pathlist; do
             fi
         fi
 
+        # Per-chain dRPC lag thresholds (in blocks) so check-health.sh reports
+        # online/lagging/syncing the same way the dRPC gateway does (homogeneous status).
+        # Resolve by drpc slug first, then by chain id; empty => check-health uses its
+        # built-in defaults.
+        lags=""
+        if [ -n "$chain_slug" ]; then
+            lags=$($BASEPATH/reference-rpc-endpoint.sh --lags "$chain_slug" 2>/dev/null) || lags=""
+        fi
+        if [ -z "$lags" ] && [ -n "$chain_id_decimal" ]; then
+            lags=$($BASEPATH/reference-rpc-endpoint.sh --lags "$chain_id_decimal" 2>/dev/null) || lags=""
+        fi
+        if [ -n "$lags" ]; then
+            export LAGGING_LAG="${lags%% *}"
+            export SYNCING_LAG="${lags##* }"
+        else
+            unset LAGGING_LAG SYNCING_LAG
+        fi
+
         # Call the health check script with RPC_URL, ref, and chain-type flag
         if $is_aztec; then
             $BASEPATH/check-health.sh "$RPC_URL" --aztec $ref