Address three deferred implementation items flagged during code review:
1. SIGHUP lock held across .await (#883): Split restart_with_addr into
merged_router_clone() + install_listener() so the async TcpListener
bind happens outside the mutex, eliminating lock contention risk.
2. Recursion depth limit for check_strings (#848): Cap JSON traversal
at 32 levels to prevent stack overflow on pathological tool params.
3. Named error type for add_tokens (#788): Replace Result<(), String>
with TokenBudgetExceeded { used, limit } for type-safe budget errors.
Co-authored-by: Claude Opus 4.6 <[email protected]>
