feat(autonat-v2): emit address reachability events

[tabcat]

Description

Closes #3472.

Adds an event surface to @libp2p/protocol-autonat-v2 so applications
can observe per-address reachability state as it changes.

The service now emits:

• address:verifying { addr } — first-ever probe of this address has started
• address:reachable { addr } — verdict reached or flipped to reachable
• address:unreachable { addr } — verdict reached or flipped to unreachable
• address:removed { addr } — an address that was being tracked is no
  longer present in the AddressManager

Plus matching getters:

• service.verifying: Multiaddr[] — addresses currently being probed
  for the first time (no verdict yet)
• service.reachable: Multiaddr[] — addresses currently REACHABLE
• service.unreachable: Multiaddr[] — addresses currently UNREACHABLE

Subscribe via libp2p.services.autoNATv2.addEventListener('address:reachable', evt => …).

Notes & open questions

• Service-local event bus (consistent with Circuit Relay v2's
  relay:created-reservation etc.), not the main libp2p event bus.
• address:verifying fires only on the first probe of an address; TTL
  re-probes of a verdict'd address stay silent (the verdict persists,
  only an actual flip emits a new event).
• For an observed address going UNREACHABLE, both address:unreachable
  and address:removed fire on different ticks — removeObservedAddr
  hard-deletes the observed entry, and the next reconcile pass detects
  its absence. The events convey separate signals (verdict vs lifecycle),
  so apps subscribe to whichever they care about.
• A reachable address can also be removed without ever going unreachable
  (e.g. relay reservation drops, UPnP supersedes an observed entry,
  transport listener stops). Only address:removed fires in those cases.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[tabcat]

@srene any feedback here is greatly appreciated.