feat!: factor DHT rendezvous into iroh-gossip-rendezvous 0.1

The partition-healing / DHT-rendezvous algorithm that used to live in
`src/gossip.rs` is now its own crate, published as
`iroh-gossip-rendezvous` 0.1.0. Bridge depends on it for swarm
discovery; the 760-line gossip module shrinks to ~75 lines of glue.

`dht-partition-healing.md` is reduced to a pointer at the new
crate's `PROTOCOL.md` — the normative spec moved with the code.

### Dependency changes

- **New:** `iroh-gossip-rendezvous = "0.1"` (path + version for local
  dev; publish strips the path).
- **Bumped:** `iroh` and `iroh-gossip` 0.97 → 0.98 (major).
- **Removed:** direct deps on `mainline`, `sha2`, `zeroize` — all
  now transitive via `iroh-gossip-rendezvous`.
- `donglora-client` spec loosened from `"1.0.0"` to `"1"` so patch
  and minor releases of the client propagate automatically.

BREAKING CHANGE: any direct users of `bridge::gossip` internals
relied on the in-bridge partition-healing types that have moved.
Import from `iroh_gossip_rendezvous::…` instead.

src/main.rs

@@ -10,7 +10,6 @@ use tokio::sync::mpsc;
 use tokio_util::sync::CancellationToken;
 use tracing::{info, warn};
 
-use crate::gossip::PassphraseKeys;
 use crate::rate_limit::RateLimiter;
 use crate::router::Stats;
 
@@ -66,7 +65,7 @@ async fn main() -> Result<()> {
     setup_logging(cli.log_only, &cfg);
 
     // Ephemeral identity — fresh keypair each launch.
-    let secret_key = SecretKey::generate(&mut rand::rng());
+    let secret_key = SecretKey::generate();
     let node_id = secret_key.public();
     info!("donglora-bridge starting, ephemeral id: {}", node_id.fmt_short());
 
@@ -74,10 +73,6 @@ async fn main() -> Result<()> {
     let radio_config = cfg.radio.to_lora_config()?;
     let retry_policy = cfg.tx.to_retry_policy()?;
 
-    // Derive passphrase keys.
-    let keys = PassphraseKeys::derive(&cfg.bridge.passphrase);
-    info!("topic: {}", hex::encode(&keys.topic_id.as_bytes()[..8]));
-
     // Create rate limiter.
     let cr_denom: u8 = match radio_config.cr {
         donglora_client::LoRaCodingRate::Cr4_5 => 5,
@@ -94,8 +89,9 @@ async fn main() -> Result<()> {
     );
     info!("rate limiter: {:.1} pps", rate_limiter.rate_pps());
 
-    // Create gossip network.
-    let (swarm, gossip_event_rx, gossip_frame_tx) = gossip::Gossip::new(secret_key, &keys).await?;
+    // Create gossip network + DHT rendezvous.
+    let (swarm, gossip_event_rx, gossip_frame_tx) =
+        gossip::Gossip::new(secret_key, &cfg.bridge.passphrase).await?;
     let swarm = Arc::new(swarm);
 
     // Start radio.