perf(install): run connectivity probes in parallel — blocked-network worst case 16s -> 8s

install.sh probed pypi.org and duckduckgo.com serially with
--max-time 8 each, so a fully blocked network cost 16s before the user
saw any useful guidance. The two probes are independent; running them
as background jobs and gathering verdicts caps the worst case at one
--max-time (8s) while the good path stays instant.

Verified live: reachable URLs 0.24s (both probes concurrent);
blackholed 10.255.255.x URLs 8.02s total (was 2x8s), warning text
unchanged. bash -n clean.

scripts/install.sh

@@ -949,12 +949,33 @@ check_network_prerequisites() {
         return 0
     fi
 
+    # Run the probes in parallel — serially, two blocked probes cost
+    # 2 × --max-time (16 s) before the user sees any useful error; in
+    # parallel the worst case is one --max-time (8 s).
+    local pids=()
+    local tmpdir
+    tmpdir=$(mktemp -d)
+    local i=0
     for url in "${checks[@]}"; do
-        if ! curl -fsSI --max-time 8 "$url" >/dev/null 2>&1; then
+        (
+            if curl -fsSI --max-time 8 "$url" >/dev/null 2>&1; then
+                : > "$tmpdir/ok_$i"
+            fi
+        ) &
+        pids+=($!)
+        i=$((i + 1))
+    done
+    wait "${pids[@]}" 2>/dev/null
+
+    i=0
+    for url in "${checks[@]}"; do
+        if [ ! -e "$tmpdir/ok_$i" ]; then
             failed=true
             log_warn "Could not reach $url"
         fi
+        i=$((i + 1))
     done
+    rm -rf "$tmpdir"
 
     if [ "$failed" = false ]; then
         log_success "Internet connectivity looks good"