Add automatic protocol fallback with backoff and silence DHT bootstrap error

When multiple data plane protocols (such as tcp-tls and wg-quic) are configured,
the agent previously attempted only the first mutually supported protocol in
served order. If that protocol failed to connect or dropped (due to NAT, firewall,
or handshake failure), ensure_links retried the exact same failed protocol
on every tick without ever falling back to alternative supported protocols.
Additionally, tcp-tls mistakenly included 127.0.0.1 in peer candidate lists,
causing peers to connect to their own local listener and fail certificate
verification (NotValidForName). Finally, BitTorrent DHT bootstrap failures
in the third-party mainline crate were logged as noisy ERRORs despite overlay
connectivity operating normally.

Key improvements:
- In crates/tsunagi/src/agent/network.rs, implement ProtocolDialState with
  exponential backoff (3s, 6s, 12s, 24s, 48s, capped at 60s).
- In ensure_links, select the highest-priority mutually supported protocol
  that is not currently on backoff cooldown. If a protocol fails, immediately
  record the failure and fall back to alternative mutually supported protocols
  without waiting for the periodic ticker.
- In crates/tsunagi-tcp-tls, remove localhost loopback from local_ip_candidates
  and capability announcements, and prevent open() from connecting to our own
  listening port.
- In crates/tsunagi-cli/src/main.rs, set mainline=off in the default tracing
  filter so benign BitTorrent DHT routing table bootstrap errors do not alarm users.

Validation on Linux:
- cargo fmt --all -- --check passed.
- cargo clippy --locked --workspace --all-targets -- -D warnings passed.
- cargo test --locked --workspace --all-targets passed (349 tests passed).
- cargo build --release passed.

crates/tsunagi-cli/src/main.rs

@@ -37,7 +37,7 @@ struct Cli {
         long,
         global = true,
         env = "TSUNAGI_LOG",
-        default_value = "info,iroh=warn,quinn=warn,rustls=warn,boringtun=warn,mainline=warn"
+        default_value = "info,iroh=warn,quinn=warn,rustls=warn,boringtun=warn,mainline=off"
     )]
     log: String,