fix: detect and warn on circular router references

[som14062005]

Problem

When a Router is mounted within itself (directly or transitively),
requests that traverse the cycle hang indefinitely with no error or warning.

Solution

• Added hasCycle() helper using DFS with a visited Set to avoid infinite loops
• In Router.prototype.use, checks for circular references before mounting a Router instance
• If a cycle is detected, emits a process warning with code ROUTER_CYCLE_DETECTED

Notes

• 11 pre-existing test failures related to the QUERY method exist on the
  base branch and are unrelated to this PR
• Zero new test failures introduced by this change

[som14062005]

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