Add real ICE restart and stable stream/component teardown APIs to rice-proto

[Copilot]

rice-proto did not have a real ICE restart path: callers could only recreate the agent, which drops selected pairs, gathered sockets, and live TURN state. This change adds restart semantics at stream and agent scope, plus the missing stream/component removal APIs, while keeping stream ids stable and preserving reusable local transport state.

• API surface

  • Add StreamMut::restart(&mut self, Credentials, Instant) for explicit per-stream restart.
  • Add random-credential restart support via StreamMut::restart_with_random_credentials(Instant).
  • Add Agent::restart(&mut self, Instant) to restart every stream with fresh credentials.
  • Add Agent::remove_stream(&mut self, usize) and StreamMut::remove_component(&mut self, usize).

• Restart semantics

  • Restart updates local ICE credentials on both StreamState and the checklist.
  • Remote credentials are cleared and remote candidates are discarded for the new generation.
  • Checklist check state, valid/nominated state, triggered queues, and end-of-candidates flags are reset.
  • Existing local candidates, sockets, gatherers, and TURN allocations are retained.
  • Consent freshness entries for the restarted stream/components are stopped and re-established only after a new selected pair is chosen.
  • Restart does not change the agent role; set_controlling() remains the role control point.

• Checklist and agent internals

  • Add explicit checklist restart/reset entry points instead of reconstructing checklists.
  • Track whether remote credentials are actually set so restarted checklists do not reuse stale credentials.
  • Preserve ancillary checklist configuration across restart: controlling mode, tie-breaker, ICE-lite/trickle settings, timing, and retransmit config.
  • Remove the end_of_remote_candidates() restart FIXME by re-arming end-of-candidates across generations.

• Stable teardown / id reuse

  • Convert agent stream storage to tombstoned slots so removing a stream does not shift stream ids.
  • Keep component slot reuse working by fixing re-add into None holes.
  • On stream/component removal, clean checklist/runtime state and emit socket removals only for resources no longer backing surviving candidates.

• Coverage

  • Add focused unit tests for:
    • restart preserving local candidates
    • credential propagation + remote credential clearing
    • end-of-candidates re-arming
    • consent freshness reset/restart
    • no socket removal on restart
    • stream/component removal behavior and id stability
  • Add rustdoc coverage for the new stream restart/removal APIs.

Example:

use rice_proto::agent::Agent;
use rice_proto::stream::Credentials;
use stun_proto::Instant;

let mut agent = Agent::default();
let stream_id = agent.add_stream();

let mut stream = agent.mut_stream(stream_id).unwrap();
stream.restart(
    Credentials::new("ufrag".into(), "a-very-long-password-value".into()),
    Instant::ZERO,
);

// Or restart the whole agent with fresh generated credentials per stream.
agent.restart(Instant::ZERO);