Router: flag redirect loops in the navigation timeline

[erkamyaman]

The problem

Redirect loops are hard to spot in the router timeline.

• The route lint already flags redirect cycles in the config (redirectTo pointing back and forth).
• The timeline shows "redirected from #N" on each navigation, so you can follow a chain by hand.

What neither catches is a loop made by guards. For example, an auth guard sends /account to /login, and a guard on /login sends a signed-in user back to /account. Each hop looks normal on its own, and the timeline just fills up with navigations.

What we need

• Detect loops at runtime: follow redirectedFrom through recent navigations and flag a chain where the same URL shows up twice.
• Show it in the timeline: mark the navigations in the loop, show the chain (/account → /login → /account), and name the guard or redirectTo behind each hop when it's known.
• Lint: add a redirect-loop finding with the chain and the guards involved.
• MCP: include the loop in explain-navigation and export-navigation, and in the lint tool, so agents can explain it.
• Tests: a guard-to-guard loop, a redirectTo loop, and a long chain that isn't a loop (no false positives).

Notes

• Guard results are recorded when router recording is on (on by default in dev), so the chain can say which guard redirected.
• Angular stops real infinite loops with NG04016; this should catch the loop before that point too, including loops that end after a few bounces.