feat: add router cycle detection to prevent infinite loops

[aprici7y]

Detects when a router is mounted on itself (directly or indirectly) and emits a warning to help developers identify cycles that cause requests to hang indefinitely.

When a router is mounted on itself through a chain of routers, requests that traverse the cycle hang indefinitely and eventually cause a stack overflow. This is difficult to debug because the application appears to work fine until a request hits the cyclic path.

This implements cycle detection in Router.prototype.use that:

• Detects both direct self-mounts (router.use(router)) and indirect cycles (routerA -> routerB -> routerA)
• Uses recursive traversal with a visited set to prevent infinite loops during detection
• Emits a process.emitWarning() with type RouterCycleWarning
• Maintains backward compatibility - the mount still succeeds but with a warning

Manual Test

const Router = require('router')

// Direct self-mount
const loop = new Router()
loop.use('/', loop) // Emits RouterCycleWarning

// Indirect cycle
const routerA = new Router()
const routerB = new Router()
routerA.use('/b', routerB)
routerB.use('/a', routerA) // Emits RouterCycleWarning

Fixes expressjs/express#6809

[IlyasShabi]

You run this function each time, so why not adding a Set/Map to avoid repetitive code?

[som14062005]

I've opened a fresh PR addressing the same issue with a cleaner implementation: #193

Implementation Details

The fix adds a hasCycle() helper function that performs a depth-first
search (DFS) through the router tree before mounting any router instance.

• In Router.prototype.use, before pushing a layer onto the stack,
  we check if the handler is a Router instance
• If it is, hasCycle() walks the entire subtree of that router using
  a visited Set to avoid infinite loops during the check itself
• If a cycle is detected, process.emitWarning() is called with code
  ROUTER_CYCLE_DETECTED, giving developers an immediate, actionable warning
• If no cycle exists, everything proceeds as normal — zero performance
  impact on the happy path