aeronet_iroh: add PathReport session telemetry and prompt path-migration updates

[baadc0de]

What

Adds per-session path telemetry to aeronet_iroh sessions, and makes
relay→direct path migration visible to the ECS world within a frame instead
of up to 100 ms late.

PathReport component

New session::PathReport component, inserted when a session connects and
updated by the IO layer alongside the existing SelectedPath/PacketRtt:

pub struct PathReport {
    pub kind: PathKind,              // Direct | Relayed { relay: RelayUrl }
    pub rtt: Duration,               // RTT of the currently selected path
    pub connected_at: Instant,       // when the session connected
    pub direct_since: Option<Instant>,
}

impl PathReport {
    pub fn time_to_direct(&self) -> Option<Duration>; // direct_since - connected_at
}

SelectedPath already exposes the raw TransportAddr; PathReport adds the
two things games actually want to graph and gate on:

1. relay-vs-direct as a first-class enum (no per-frame matching on
   TransportAddr variants), and
2. time-to-direct-path — how long hole punching took. This is the key
   NAT-traversal health metric for peered sessions (iroh's own baseline is
   ~90% direct); without it you can only observe the current path, never
   the migration latency.

Prompt migration updates

The backend meta loop previously polled path state on a fixed 100 ms
interval, so a relay→direct migration could surface up to a full interval
late. It now also wakes on Connection::path_events()
(PathEvent::Selected/Closed), so PathReport, SelectedPath, and
PacketRtt update on the next frame after a migration.

This needs no new dependencies and works on WASM too: PathEventStream is
not a fused stream, but StreamExt::fuse() wraps any stream into a
FusedStream, so plain futures::select! can poll it.

test-utils feature

New optional feature exposing test_utils::run_test_relay(): an in-process
loopback Iroh relay (HTTPS + QUIC, self-signed certs via
iroh-relay/test-utils) returning (RelayMap, RelayUrl, Server). Lets
downstream crates write offline integration tests that exercise the real
relay path — including hole punching — with no external network.

Used by the new tests/relay_path_report.rs: two endpoints over the
loopback relay connect, exchange datagrams both ways, and assert
PathReport populates with a sane path kind, non-zero RTT, and (when a
direct path is selected) a time_to_direct.

Why

We're building a P2P multiplayer stack on aeronet + iroh
(Orrery) where relay-vs-direct path
and time-to-direct are first-class session telemetry: the coordinator uses
them to size islands and to detect degraded peers. Every P2P game on this
transport will want the same signals; they belong in the IO layer.

Compatibility

• Purely additive: new component, new feature (default-off), no changes to
  existing public API or behavior (besides the faster telemetry updates
  noted above).
• No new required dependencies — the prompt-update path uses only futures,
  which the crate already depends on, and works on WASM.
• test-utils pulls in iroh-relay with server + test-utils features,
  only when enabled.

Validation

• cargo clippy -p aeronet_iroh --all-targets --all-features -- -D warnings — clean
• cargo test -p aeronet_iroh --all-features — all pass (existing
  session.rs tests, new relay_path_report.rs, doctests)
• cargo doc -p aeronet_iroh --no-deps --all-features — clean
• cargo build -p aeronet_iroh --examples — clean

[aecsocket]

re test_utils, would rather have that as a separate file in tests that you include! or treat as a mod, rather than a cargo feature