Add gather generations and tag candidates - #945

Follows up on #874 by @boushley, carrying the same goal forward with a restart-scoped implementation aligned with libwebrtc.

Tag every gathered candidate with a generation extension holding the
candidate's gather generation id — the attribute libwebrtc-based
browsers emit so peers can tell candidates of a restarted gather cycle
apart from the previous cycle's.

Two commits:

1. Encapsulate ICE generation state in one type. Local credentials
   and gathering state were separate Agent fields that a restart
   updated piecemeal — an in-flight gather cycle could read a
   credential the restart had already rewritten, a data race. The new
   iceGeneration type bundles the generation id, credentials, and
   gathering state; Restart swaps the active generation on the event
   loop, and a gather cycle reads credentials only from the generation
   it captured at start, immutable for the life of the cycle.
2. Add the generation candidate extension. A superseded cycle
   tags its own generation, not the agent's current generation. Only
   an ICE restart advances the generation — a continual-gathering
   re-gather reuses its cycle's generation, and the initial gather is
   generation 0 — matching libwebrtc.

Behavior notes

• Marshaled candidates (and downstream SDP a=candidate lines) gain
  one additive generation N token; peers that follow RFC 5245
  extension parsing ignore unknown extensions.
• Close joins a gather cycle that a restart superseded when no new
  gather followed; restart-then-regather leaves the previous cycle to
  context cancellation, unchanged from current behavior.

Testing

• Unit tests: a superseded cycle tags its own generation and ufrag; a
  canceled cycle's state write is dropped with no late
  end-of-candidates; Close joins a superseded gather. A vnet test
  asserts generation 0, then generation 1 after a restart.
• Full suite green under -race; golangci-lint clean. End-to-end
  against pion/webrtc main (local replace): SDP candidate lines carry
  generation 0, then generation 1 after an ICE restart.

Review focus

• Add gathering generation extension to candidates #874 bumped the generation on continual-gathering network changes;
  this PR advances the generation only on ICE restart, matching
  libwebrtc (a generation is an allocator session, created only on
  credential change — p2p/base/p2p_transport_channel.cc). For the
  flapping scenario discussed in Add gathering generation extension to candidates #874, libwebrtc prunes failed-network
  ports and relies on connectivity checks rather than generation
  filtering; equivalent cleanup is out of scope for this PR.