fix: forward wrapper shutdown signals to Next child

[NgoQuocViet2001]

Summary

• forward SIGINT and SIGTERM from the pi-web wrapper to the spawned Next.js process
• allow up to five seconds for graceful child shutdown
• force-stop the child when shutdown times out or another termination signal arrives
• preserve child exit codes and conventional exit statuses for known signals
• cover signal escalation, timeout cleanup, listener cleanup, and exit statuses with regression tests

Validation

• Ran: node --experimental-strip-types --test lib/process-lifecycle.test.mjs
• Ran: npm test (560 tests passed)
• Ran: npm run lint
• Ran: node_modules/.bin/tsc --noEmit
• Ran: npm run build
• Ran: npm pack --dry-run --json
• Checked: a Unix SIGTERM smoke test exits the wrapper with status 143 and leaves no child process
• Checked: a long-lived HTTP stream is force-closed after the five-second grace period; the wrapper exits with status 137 and leaves no child process

Fixes #495