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/packet.rs

@@ -144,7 +144,7 @@ mod tests {
 
     #[test]
     fn gossip_frame_roundtrip() {
-        let key = iroh::SecretKey::generate(&mut rand::rng());
+        let key = iroh::SecretKey::generate();
         let frame = GossipFrame::new(&key.public(), -85, 8, vec![1, 2, 3, 4, 5]);
         let encoded = frame.encode();
         let decoded = GossipFrame::decode(&encoded).expect("decode should succeed");
@@ -235,7 +235,7 @@ mod tests {
                 snr in -32i16..=32i16,
                 payload in proptest::collection::vec(any::<u8>(), 0..256),
             ) {
-                let key = iroh::SecretKey::generate(&mut rand::rng());
+                let key = iroh::SecretKey::generate();
                 let frame = GossipFrame::new(&key.public(), rssi, snr, payload.clone());
                 let encoded = frame.encode();
                 let decoded = GossipFrame::decode(&encoded).unwrap();